To avoid a big mess and lots of clean up I used a deep tub with only a small layer of baking soda covering the bottom. I broke up any baking soda chunks with a kiddie fork (that he can later use to play with if he wants). I dyed a small cup of vinegar blue because that's what Dean chose - he wanted to make blue snowflakes.
I gave him a small medicine dropper and showed him that a few drops of the liquid creates blue bubbling mountains! He was ENTHRALLED! After about 45 minutes he dumped the whole cup in the bin and played with the slushy putty it made.