Fizzy painting is just as simple, and possibly a little less messy than volcanoes and rocket ships. All you need is heavy duty paper (construction paper or poster board will work fine), glue, a little food coloring, paint brushes, and of course baking soda and vinegar.
Start by painting your design on the paper with glue, and sprinkle baking soda over the design (like you're making a glitter project).
Allow that to dry while you work on puzzles, squish play-dough, have a snack, or whatever other activities you need to fill up an hour of time.
Once the glue has dried, mix about a tablespoon of vinegar with a few drops of food coloring. I wanted a bold color, so I used about seven drops of purple food coloring.
Using your paint brushes, carefully drip little bits of your vinegar mixture onto the baking soda design.