Rubber bands, known weights
Wheatstone bridge, resistors, various materials Rubber, metal, steel wool Electrochemical Cells High voltage, low capacitance "human" circuits. Parallel vs. series.
Simple Solar Cookers Temperature equilibrium Energy of common foods
Multi-subject projects Making a better mousetrap...car that is. Floating boats: making boats out of balsa and seeing who can get trough the 20 meter float. The green cannon: Using renewable energy to accelerate a tennis ball
Computer speakers Representation of waves Annoying music, characteristic waves.