Last weekend, the girls and I decided on a spur of the moment trip to a physics fair at a nearby university. It was crowded, but a lot of fun and the girls got to do a lot of hands on experiments which were all explained by college students and some professors that were manning each station.
Here's Sedona walking on water (and cornstarch):
One of Sedona's favorites was getting to safely check out the sun and learn a little about sunspots. She loves all things telescope (though I sometimes wonder if that's because using a telescope generally involves staying up late).
University of Minnesota site. They do some of these same experiments in shows. There are videos of some of them and if you click on the heading of each section, it will explain why the experiments work the way they do.