Simply lay down some towels, fill one or two large bowls with water and add a few toys from around the toy bin (or living room floor as it may be).
We started off on the kitchen floor with one bowl filled with a few cups of water. Both the 10 month old and the 2 1/2 year old enjoyed using left over Easter eggs to scoop and splash around in the water.
After about 15 minutes, the baby continued to splash happily but the 2 year old was looking to up the excitement. No problem. We got out another two bowls for him and filled the first about halfway full. Then we added a fun sponge ball, some favorite trucks, and a hand towel. Suddenly we had a car wash.
We were actually introduced to this activity at a friend's house. We started with a bowl full of various toys and conducted "float or sink?" experiments. Eventually that morphed into practicing pouring water from one bowl to the other.
These are only a few examples of the fun you can have with bowls of water. Suggest the idea to your kids and ask them to find items to play with in the water. You'll be surprised with how much they can come up with!