Hey i have a toddler myself, and i know what you mean!
You want to make it more grown up and colourful but not not spend alot of money!

If your in australia, you can go to spotlight, they have packs of kids furniture, such as say a toddler bad and some cupboards.Or just the beds, in characters, and they are reasonably cheap! It will last him till he is big enough for a single bed! Also they sell like tables and chairs!
But if you do not wish to go out and buy this new, id say just go to target or a store thats similiar and just buy a clip on rail. I think they are like $30. (or ebay)

Go to your local baby stores, and check out thier sales on decor for rooms. For my daughter i just picked up a pack (think there was 6-7) wooden decorations that you just stick on the wall. They are like princess theme, so castles and so on, they arent big maybe like 10cm x 10cm... and i think that was like $20. I dont know where your located (please tell me =]) cause i got these at baby kingdom! (Im in australia)
But alot of baby stores have sale bins on decor for rooms. You can usually find alot of nice things cheap!

Or if you really dont have time or money to search. You can just get old wrapping paper (that is in good condition) with nice prints for boys such as cars and so on (if you have non just go to a dicsount store and buy a sheet for $1!) and frame these. I have in my daiughters room, in a frame, the first wrapping paper she ever got from a gift when she was born. Its soo cute and is sentamental aswell! You can do this aswell with gift bags you have laying around!

Or you could simply get your son to draw pictures of his favourite things on bright coloured paper and frame this! And when he has friends or family over he can show it off!! Lets him contribute!

Hmm think thats all i can think of at the time sorry lol, we did so many things like this, not just to save money, but so its unique and different!

hope its helped!
goodluck!!
I will add any more ideas i come up with! =]

You know what's better than buying stuff?
Making it! I love doing crafts and I'm sure he would too. What is his favorite thing to do? If he loves to play with spiderman then have him paint a picture of spiderman and put it on his wall. Even if it isn't in a frame you can just put the paper on the wall. What you can do also is take pieces of paper taped together and write a quote on them in blocky letters like "A baby's gotta do what a baby's gotta do" (rugrats) and maybe change it to boy or his name. It's cheap and fun. Then put it on the wall at the top. You can also take furniture you have and spray paint it white or another cool color to make a theme, look in different rooms. And keep on looking on Craigslist for more furniture. There are a lot of options!

Look on the bed you have for a model # or on any assembly brochure that came with it. If you know the manufacturer of the bed, you can look them up on the internet and call them to order the side rail or one that will work.

Two gal. of paint, even at $25 a gal. will paint any 11 X 12 room in two coats of paint. Check Home Depot, Lowe's etc. I'd use the wall decal Wallies from the link below for a changeable wall decor theme. I've linked you to the clearance page where the items are super inexpensive.

Use a simple small bookshelf and a corner toy hammock which you can find for less than $59. Go to Second Time Around stores to find very gently used children's bedding, the toy organizer perhaps and lots of other kid's things for the room. If they have a group called Freecycle in your area, then you find things on their site for free and they're not junk, either. Most of the things are from people downsizing or redecorating. Good luck!! : )

Sales. Go slow.

For instance, with paint, go to your local paint store. Not Wal-mart or something like that, but some place that specializes in paint. Ask them about mis-matched paint that people don't want anymore. You only need a gallon. What happens is someone comes and brings a sample and they get home and look at the paint and say "That's not the color" then bring it back. The store will usually take it back and remix a color, and then sell the gallon they didn't want for 1/2-1/4 of the price. :)

Then just go bit by bit. The wooden toddler bed, watch for sales and keep going to the goodwill and such. I found a toy-box for my daughter that is hand-made wooden from a specialty shop in California (we're in Colorado). The top cushion is faded, but it's in great condition. For toy organizers, hit the after school sales, ie, right now they're doing "back to school" but soon all that will go on sale as they make room for Halloween. You can get all sorts of organizing shelves and boxes at that time for cheap. Watch the fabric stores for that stuff too. Also after Christmas is a great time to watch for these things.

Some of these things you can put on the Christmas list for your child too. Like the posters, bedding, or the table-chair set. Hit garage sales too. I found my daughter a little chair that she loves (and is in very good condition) in a trash-can. I also know that Wal-mart does plastic chairs and tables around summer-time. If your local wal-mart still has them, you can pick up the chairs for about $5 a piece (we got ours for $4 on the clearance, they're normally $6) and the table for like $12. If you can't find it now, wait until summer next year.

For posters, make your own. :) Fingerpainted posters look great and the child feels really proud of their own work. At the same time, I've found all sorts of art for under $5 at my local ARC. One, when I got home and looked online to find out what it was worth, was a $30 lithograph with a heavy frame... so worth a lot more than $4. And one, which I love, is probably close to $60, I paid $8. You have to dig around in the back, and know which stores do the best sales on that (for me, it's a Goodwill store, and the ARC. I know that Salvation Army where I live doesn't do a great deal on art.)

I hope this gives some ideas.

If you're struggling with bed options, you could try a Montessori bed for a while. It's essentially a floor bed, but the point is not just because you don't have a bed; it's because it makes it a whole lot easier for a toddler to climb in and out, plus you don't have to worry about falls or anything. I promise there is actually research behind it and it's not just some "throw the mattress on the floor", even though it is a mattress on the floor. Then, you don't have to worry about guard rails, matching metal frames with the room, or anything else.

Really, though, I'm wondering what in the heck you think he needs a table and chairs, toy organizer, poster, and bedding for. His bedroom is primarily for sleeping, so you can nix the table and chairs. Maybe try a beanbag chair or a couple of body pillows in a corner and a travel lap-desk thingy to set up a reading/coloring/whatever area.

Spend $6 on a big rubber-made tupperware container (you can pick different colors to look cooler) to use as a toy box. Or, buy several smaller ones to organize the toys. Really, a toddler doesn't care too much about organization, so two or three of the smaller ones should be enough to distribute the toys and not have a mess. We even have some gallon sized ice cream buckets that we use, and our toddler loves them because they have handles he can use to carry them around.

Since a toddler bed mattress is the same as a crib mattress, I'm not sure what kind of extra bedding you'd need. If you can just use the crib mattress you have to create a nice floor bed, you can use the same sheets. We've had several quilts that were baby gifts that we pulled out to use once he moved out of his crib. He still sleeps on them, though, since he'd rather snuggle underneath his favorite soft blankets. Nix any bedding that would be just for show and get a good pillow with a couple of cases (bright solid colors are really versatile), maybe a couple of new fitted sheets (a toddler isn't going to stay under a flat sheet, and their blankets get washed very regularly anyhow, so it's no big deal), and keep using his favorite snuggly blankets.

As far as walls and decorations go, maybe you can keep it simple with just a few things to make the room fun. Besides, you don't want to over do it and make the room too stimulating for him to sleep in. You could use his own art work to hang up on the walls. You can even color and finger paint some fun artwork to hang up for him.

Do you live near a military base? Here's a little secret, lol...most military bases have their own craigslist! All you have to do is put in www.nameofbaseyardsales.com IE: for Fort Lewis WA it's www.lewisyardsales.com. It's PCS season so lots of people are moving - see if you can find anything at the closest military base to you.

Things you can find and prices will depend a lot on where you live. Have you tried "Freecycle"? It is in the yahoo groups section. and you put in the area or state you live in. You post things you are looking for and if anyone has it they let you know. Plus I found lots of things we needed just by going through the ads.

Paint is a big expense, but the benefit of paint is it will last the longest, and change the feel of the room more than any thing else. Plus there are a lot of paint techniques that can make it even better. Say painting the bottem half a deep blue or green, and then the top half a lighter shade of the same color family. Or paint wide vertical stripes. and you only need a roll or two of painters tape.!!

You can even look on freecycle for paint. !!!

I have seen toddler bed rails for much cheaper at stores. you just have to look around.

You will have to just keep looking. goodwill and other stores often get new things in. so just keep checking back with them.

We just painted my toddler son's room, but we only did the bottom half of the walls. We used a self-adhesive wall border in his favorite theme (Diego) to separate the bottom and top color. Then we put other self-adhesive stickers in the Diego theme all around the room. The stickers were not expensive at all and we ordered them on the internet.

We also have a 3:1 crib to toddler bed for my son. We don't use safety rails, just a big pillow on the floor in case he rolls out.

Don't go too crazy on decorating to perfection since he'll/she'll will be into another character or theme in about six months. We might be switching to Disney's Cars theme shortly since that seems to be his new favorite thing. Just have to buy new stickers and we're good as done. Good Luck!

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My biggest cheapie when decorating my daughters' room was window treatments. Instead of paying a bunch of money for curtains, I went to Walmart to the sewing section, went through the $1 a yard material and purchased yards of three solid colors....I used decorative window ends and pulled the material through them and let it hang to the floor on either side and it looked great. I was out of $25 total I think.....