This great idea comes from my co-teacher. She originally thought to let the kids paint on the inside of one of our windows since in fair weather we let them paint on the outside. After thinking about it she decided to use our one-way mirror instead. She covered over the edges with newspaper and added a step. In the morning we added paper underneath the mirror because the step wasn't tall enough for every child to reach much of the mirror and we added a platform of large blocks just to make the standing area larger.
I suppose with some extra time we could have figured out how to rig up the paint on the wall (any ideas that don't involve damaging the wall?), but for today we simply put flat, wide cups of tempera paint out with various brush types. After the painting seemed to be done, we walked the children around to the other side of the mirror so they could see what their artwork looked like lit up from our classroom lights.
In the picture you're seeing the reflection of our room. I'm not sure who put up the fish hanging from the ceiling, but it's been a hit for years with the kids. It doesn't make for great photos, though.