Some of the ideas I find out there take forethought & planning...but others can be done spur of the moment. This was one of the latter: perfect!
Line the bottom of a pan with baking soda. Grab some vinegar; white vinegar is best but apple vinegar works, too. Pour the vinegar into small bowls or dishes & tint it with food coloring. I kept it simple: blue, yellow, red, and green.
The photo on Pinterest showed their toddler using an eye dropper. I didn't have one of those, so I improvised. We used a bulb syringe and that worked just fine.
Jonah got the concept right away: filling the syringe with liquid & squeezing it out into the tray. He enjoyed watching colorful volcanoes erupt and bubble all over. This was simple, yet great sensory fun for him!