Ads in the paper copy, available at Borders Books, are good sources for parts and ideas. Adventurous 5th graders might find something of interest in the articles, too.
Lots of ideas from the K-12 list, some of them followons to the Lawrence Berkeley Hall of Science Night...many very appropriate for K-5.
A fun project from Spark Bang Buzz, good for 4th grade. Explore the site for more inspiration