I saw this on pinterst, but can't track it back to a clickable link. Sometimes things there are just pictures taken from an iphone, so there's not a real source. Anyway, baking soda is spread out in a tray and vinegar and food coloring are poured into cups.
The kids use eyedroppers to pour small amounts of the vinegar on the tray. There is the usual vinegar/baking soda bubbling and lots of color mixing.
It only took my kids a few minutes though to take it to the next level. That's the messy level.
They figured out that if you added the baking soda to the cups, there was even more fizzing.
They decided that If your whole hand got involved it got even more fun.