Scitoys.com has an impressive array of science gadgets that kids and adults will enjoy making.  The website even has an online store that sells everything you need to make some of the gadgets.  However, your local RadioShack should have much of the items needed.  What sets this website apart from most other science websites for kids is that many of these projects support learning advanced science and engineering concepts.  For example, there are lessons on building your own hydrogen fuel cell, extracting DNA in your kitchen, and a simple (but cool) electric motor.  Additionally, Scitoys.com always explains the physics, chemical, biological and mathematical principals behind the devices.  So, if you are looking for real science engagement (beyond the tired old vinegar-baking soda volcanos), build a few science gadgets and learn some really advanced science concepts along the way.