There's always that sticky tack putty stuff. We could only use that on our walls at the UO dormitory. It works really well, and so long as the walls aren't made of paper or covered in fabric, there shouldn't be any blotchy marks left over.
Perhaps if you need one room to be filled, it can always end up as a drawing room or an art competition? I'm game for that. I love seeing people's styles and things like that. If we get enough putty, it'd be interesting to see a wall grow with finished artwork. In Kumoricon '04, there was a drawing table where you could take a sketch you like and leave a sketch. It was pretty neat, I thought.