I did my daughters room with finding nemo/ocean theme, I like winnie the pooh but everyone has winnie the pooh and I like blue as well as nemo lol!!!
For a boy another thing to try is moons/stars, safari or even noahs ark etc, if your painting I like two colors with a wall border add extra class lol!! If your renting you may want to look at painting the whole room one color.
Once little one is born my daughter is moving into her own room im going to do hello kitty, there is not much stuff of hello kitty so alot is going to be either handmade or painted.
Its up to you be creative and if you hit a block check some sites out and if you see something you like copy it on your budget, I love sewing and stuff so it fun to actually create something.