Nasturtium leaves are very cool to mess around with. Pick a large nasturtium leaf, and try to catch some water in it. Something about the surface of the leaf resists the liquid completely, so that the water forms into beautiful round silvery drops that race around and off the leaf before you know it. They behave like mercury (have you ever broken a mercury thermometer?), but are not poisonous like mercury.
Catching the water is fun itself, but then you can try to see how big a drop your leaf can hold. Or see how many small drops its possible to accommodate on your leaf, since the moment two drops touch, they become one bigger drop. Or you can run races and see who can go furthest and fastest before your drop slips out of the leaf.