This was definitely a summer, messy activity; we froze tempera paint to use outside! You put 1 tsp of tempera paint in each section of an ice cube tray and fill the rest of the way with water. Stir it with a toothpick, let it sit 10 minutes, and then stir it again. Then freeze until solid. Use (outside) to paint on thick watercolor paper. It takes a little bit to really see results as the paint has to begin to melt, but after it starts it really goes!
We also learned tempera paint still stinks when it is frozen and does not come out of clothes if they are not washed immediately, so make sure you wear something you don't mind paint on.