Baking soda & vinegar!
My kids love to see the reaction between baking soda & vinegar. It never gets old to them. So, I bought a big box of baking soda, mixed food coloring with vinegar, got medicine droppers, and let them go nuts!
This kept them busy (and quiet!) for awhile. They just kept adding more baking soda as they needed it. I love that they are amused by cheap and easy things sometimes. :0)