This is a great activity which can be spread over a few days.
Day 1 get outdoors and go on a big scavenger hunt. Allow your child to pick up whatever they want, like pine cones, rocks, stones, twigs, flowers, sheep wool, anything! Make it into a game and ask them to find items and make it as easy or as hard as you like.
When you get home, place them in a plastic lunchbox and fill with water and pop the container in the freezer.
Day 2 is to unfreeze your treasure.
If you have a toy hammer you can use this to smash the ice. You can watch the sunshine melt the ice explaining how warm the sun is. You can use warm water to pour over OR salt water and a calpol syringe.
This is a brilliant little experiment for them. The salt melts the ice super fast to reveal the previous days finds.