Removeable wall stickers are excellent. It's all I have on my kids walls. They also do not take any paint off. It's a great way of decorating as kids change their minds so often.
You can get them from places like Baby Bunting, Ebay. If you google removeable wall decals, you will get heaps of places that sell them.
I just got some for DD from an online shop called Giggle Smile Designs. Much cheaper than at a store.
Bambini Pronto is an online Baby and Maternity store and they have a beautiful range of everthing.
If you can't put picture hooks up, use the command adhesive hooks. They work well and again, don't leave marks on walls.