Fortunately today there are these great things called 'Home Stickers' put out by Novelles Images, which I just stumbled upon and I thought I'd share. I don't know what they look like it real life, but in the pictures they look pretty cool. I think that they would be great for a nursery or if you have an apartment or dorm room where you're not allowed to paint your walls. They claim to be easily removable, without leaving a residue. Anyway, if we do end up having kids, I'm definitely using these. You can buy them at changeyourwalls.com or there also some on ebay called 'Art Wall Project' and some on Etsy.
These are some pictures of some interesting ones:
You can even get them for your laptop or car :)