have you tried wall decals?  

I've never used them but my daughter did just a month or so ago in my granddaughter's bedroom - she did 'Dora' which is big with the little girls these days. Only problem we've found so far is my granddaughter, 2, has pulled off several that were in her reach so if doing a 'kids room' you may want to place them out of reach.

I only used these when my children were small to decorate their walls. I found that if you moved them alot , they would lose their "cling" so you want to make sure you place them where you want them the first time. Other than that, they were a great way of changing the walls without having to paint or stencil anything.

Those for the toilets I have tried before. And they are not that easy to remove!

With the new paints like wash and wear, matt and deluxe that are commonly used these days, I am sure the decal will take some of the paint with it when you remove them for the next change. Be prepared to do some minor plastering and repainting of that wall you want to change when you use the decals.

There are some really interesting designs though. Heh heh... with money to spare, using the decal will be a quick fix for a plain wall.
The other option is to work your creativity muscles and paint the patterns on the wall yourself!
I am still trying to come up with a nice design, and hopefully can convince my wife to let me paint them on one of the walls in my home ;)