In our chemistry unit, when we study States of Matter, our most popular activity is always ice excavation. It takes some time but little effort to prep, and can keep kids busy for a long time!
Prep: fill a big container with an inch of water. Freeze that. Then put in some toys or trinkets and another couple inches of water. Freeze that. Then another layer and another until you have a whole container of ice with cool treasure suspended in it.
Activity: Put the ice in a big tray that will hold the melting water. Add dishes of salt and spoons. Add containers of water with pipettes. Add tools – maybe a fork? Or a chisel and toy hammer or an icepick for older kids. Tell them their job is to try to excavate all the items.
You could also let them use a hair dryer to melt the ice, if you’re there to supervise it, since there’s a risk of dropping the hair dryer in water from the melting ice.