One explanation of the laser pointer sound experiment
Well organized list of ideas by subject (electricity, math, life sciences, etc.)
A fun project from Spark Bang Buzz, good for 4th grade. Explore the site for more inspiration
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.
Some ideas here for 3rd graders who have studied sound
Great resource for parts and kits
Good for both early grades and grades 4-5, a variation on the old "electromagnet nail" and uses a Slinky to boot
Very relevant, and fairly easy to do; uses the Internet, and direct observation, and statistics (4th grade)
Much more fun and easier than it sounds; never fails to impress
