This was part of our Water Cycle box and is actually a microwavable chicken roaster! So you may want to use it for that too!
Place rocks and tufts of grass inside along with a dish of water and put the lid on tightly (tape over vents). Place in the sunshine or in a very warm spot and over the course of a day or two, the water from the dish will start to gather on the clear lid. When you tap the lid, it 'rains' back down.