An oldie but goodie. We used a pipette and a salt shaker bottle to transfer vinegar into a tray of baking soda. (I use Colorations liquid watercolors instead of food coloring because it doesn't stain).
J was delighted with the bubbles it made. He said, "bubbles" and "izzy" (fizzy). Eventually we had to refill the vinegar and replace the baking soda (three times). Then we reversed the process and added baking soda to a cup of vinegar. He liked this process better.