So tonight I learned about Wall Tattoos and Wall Decals, which apparently (at least in some cases) are removable and therefore a good option for renters.
Here are a couple links:
DeZign with a z
Not in my budget, but fun to look!
Well, apparently there are some on Amazon too. Maybe there is a way to work something fun into the budget. Here are the ones with the best reviews there, in the "restricted budget" range: