We tried out an altered version of the water/squirt gun painting. They suggest to spray on some fabric, like an old sheet or something. I rebelled and went with paper. It didn't work as well as fabric would because it doesn't absorb, but just sort of runs off the paper.
We don't have tempura paint, so I just used some washable paint we have by Crayola and mixed it with water in some cheap water guns bought specifically to paint with (in case they got gunked up). You should be able to find these pretty easily. I got 3 for $.99 and I bet there are even better deals out there. As you see in the picture there are 4 guns there. One was an old one that we had and ended up not working.
Before we started I had the kids draw with crayons on the paper. I figured since the paint would be thin on the paper that this would work sort of like water colors over crayons. (It wasn't as cool, but I think our paint was too thinned out.)
Using the hang the paper on the fence method, we attached the kids' paper to the fence before they started painting.
And then they went to town squirting paint on the papers and loved it.
It certainly was too runny, but it was fun, so it didn't matter that they didn't turn out pretty. I want to try this again with more proper supplies.