This is a great activity for a warm sunny day. First we filled a couple of muffin trays with water. We put food colouring in the water, mixing up a slightly different colour for each hole (although the colours were difficult to see against the dark muffin tray, it did make them prettier in the water). We then collected a variety of small things from flowers and leaves to plastic toys, old shells and coins, and dropped one in each hole.
And then for the most difficult part of this activity: waiting for it all to freeze.