My go to these days (as I'm sure for you too) is pinterest. I found this fun pin that included baking soda, vinegar and food coloring( I only had gel, but it worked find)...super easy!
I chose a small container to fill with baking soda, taught The Buddy how to use a dropper (given to me for free from the pharmacy!) and filled 3 small bowls with vinegar and food coloring.
He went to town dropping the colors into the baking soda and watching it fizz.
He tried so hard to color all of the white parts. If I had given him a bigger canvas, he might has continued for a full 30 minutes! We will definitely be doing this one again! Let me know if you try it out :o)