Force

Rubber bands, known weights

Electricity

Wheatstone bridge, resistors, various materials
Rubber, metal, steel wool
Electrochemical Cells
High voltage, low capacitance "human" circuits.
Parallel vs. series.

Heat

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

Sound

Computer speakers
Representation of waves
Annoying music, characteristic waves.