Science+Fair+Ideas

http://www.all-science-fair-projects.com/category25.html

This site has earth science (geology) projects. You could even do the volcano, earthquake, or glacier demo. as a SF project. For volcanos, you could talk about different ones kids bring in and see which way to construct or materials used to "explode" work best.

Also, we aren't there yet but there is that experiment where you strain water through different soils to see which retains water. You could do it early (since we'll be doing it later) and use that as well.