Well, somedays just aren't great for being outdoors. Maybe it's too hot or muggy or rainy. That's a great day for a budding scientist to grow some crystals. It's simple and you only need a few things...
All you'll need is:
Two glass jars
Alum (found in grocery stores in the spice section).
Then you'll need some patience as it takes a few days for the crystals to really grow. Find the complete instructions over at Scribbit.