I found this super fun idea at Quirky Momma and knew my kids would really love it.
We mixed up a container of baking soda with 1/2 cup of cornstarch. Then we added about a cup of hot water. I separated the mixture into two containers and had the kids add which color they wanted. T wanted red and R wanted purple. We poured vinegar into their spray bottles and headed outside. They got busy painting the cement which was fun all by itself. When we began to spray the vinegar, it got really exciting! By the end of our experiment, they were exploring it with their hands and feet. I kept a tub of clean water nearby for washing hands since my son isn't big on getting messy. We used the tub of water to clean up when we were done and it simply washed away. Can't wait to do this again now that the weather is getting warmer.
Thanks Quirky Momma for the inspiration!