Arvind Gupta's site "Toys from Trash" is a great learning resource for teaching kids science. It's been at least 10 years since I found this site and he has all kinds of simple, inexpensive projects for elementary school kids to do. But that doesn't mean big kids can't use them also.
Here are some of the topics and projects they have: pressure, water pump, simple motor, simple generator, putt putt boat, AC motor, DC motor, air and water, beginner biology, force fun, fun with light, strong structures, etc.
Here's his portable generator:
Here's his LED torch:
Source
Toys from Trash.
