What shall we put in the bath?

This went down a storm with my 3-4s today :)
This month's theme is 'daily activities' so we started by talking about bathtime. Then we tested different bath products to see which made the most bubbles.
I put about an inch of water in the bottom of some 500ml clear pastic bottles, then added a little bubble-bath, shower gel, shampoo, conditioner, toothpaste, bath salts etc. We talked about which ones might make good bubbles, then I got the kids to choose which they wanted to test and blow through straws into the bottles for a minute or so. We then lined up the bottles and compared how high the bubbles had risen, and decided which substance was best to make a bubbly bath.