This is a great one for hot summer days. Find some small plastic toys (we have a stash of dinosaurs, bugs and frogs from Kmart) and put them in ice cube trays filled with water. Pop the trays in the freezer and have them ready for a warm (or cold) day when you need to keep the kids occupied for a while.
The object of the activity is to get the toys out of the frozen ice. I give my kids plastic eye droppers and they suck up warm water from cups (which I usually colour with food colouring) and pour it all over the ice to make it melt. I also give them a small cup of salt and let them sprinkle it over the ice. It's very interesting to watch what happens.
In summer we take our ice cubes outside and play in the sun. In winter, I still get requests for the ice game, but we do it inside and put the ice cubes in a container to catch the water when it melts.
Super fun and the kids will be enthralled!