This was a fun one for my 2yo, but also managed to captivate my 3yo and 6yo for a bit.
Dump some baking soda raisins and vinegar into a jar and wait. The bubbles fill the crevices of the raisins and make them float (like water wings on your baby). Then as they rise to the top, the bubbles pop, the raisin sinks, and it starts all over. Fun to watch.
Don't let your raisins soak for too long after the experiment. Vinegar and soggy raisins are not pretty.