I have been wanting to do the ice and salt experiment for a while now with the kids, I was just waiting for the weather to warm up a bit. It's a perfect activity for a warm spring or summer day. Last week we had a lovely day, and I had remembered to freeze a nice big block of ice.
It was one of the nicest, simplest and most satisfying experiments I've done with the kids. All you need to do is freeze a big block of ice, make a few different colours with food colouring in water, and fill a small container with table salt. Take it all outside on a tray with a couple of spoons and let the kids play.
At first it was interesting watching all the colour mixing. That kept going for a while, watching the colours blend and the ice slowly melt away. By the time the colours had mixed into a dark green colour the tunnels were looking very interesting. We could scoop some of the used water from the tray and pour it back into the ice, until we had a couple of nice tunnels, and the kids finally had had enough.