Not everything here is doable or safe for elementary school, but many are, and there are scaled down versions of others. Kids will like reading about the stuff they can't do.
A fun project from Spark Bang Buzz, good for 4th grade. Explore the site for more inspiration
Well organized list of ideas by subject (electricity, math, life sciences, etc.)
One explanation of the laser pointer sound experiment
Explore this weblog deeply for all sorts of year-round ideas, science fair included.
Power a super-bright LED from a low-voltage battery with a few bits of wire and a toroid core
Metrologic
