While not as "exciting" as rockets or volcanoes, I like the simple static electricity experiments:

1. confetti paper and comb (each kid can comb hair--buy them each one so they don't share--and place it close to small pile of confetti on table)

2. empty soda can sideways on table and balloon (statically charged by rubbing it on sweater)

I also like the forces of motion experiments (centrifugal force, center of gravity, friction, etc.)