I first posted about this activity 3 years ago and guess what? It's still fun!
First I secretly raided some bits from the bottom of the toy chest that wouldn't be harmed by some water and the freezer (e.g. marble, die, plastic animals). Then I plunked each item in an ice cube tray and added water. I got all rainbow on it this time. Last time I added food colouring which is also an option (but heed my warning on previous post).
One by one Spud excavates treasures and saves animals. Which keeps him busy while soaks. So everyone is happy.