Classify them into two groups. Those that work with electric circuits and those that don't. Or define your own groups. Bridge Building compare different ways to support a bridge. Design an Experiment design and conduct a control experiment testing for one variable.
Weather demonstrate understanding of the water cycle.
Experiment with rain and acid rain. Forensic Science learn to make, take and classify fingerprints. Encyclopedia Brown's Book of Wacky Crimes by Donald J. Sobol. Ginn 720 Reading Series. A Lizard to Start With.