Airplane from polystyrene fruit/meat plates. Cheap quick fun.
Step 1: Cut the polystyrene
Cut the edges off the polystyrene plate so that you have a flat rectangular piece.
Step 2: Draw and cut out plane
Draw and cut out your plane. Use approximate dimensions as per picture. Experiment with different shapes and sizes. You may be inclined to do some reading on flight (use it, don't use it).
Step 3: Put together and balance.
Take the cut out bits and slot them together. You can use tape if you have cut the slots too big.
Balance the plane by putting some clay/paperclips/stuff on to the nose. Make a 'V' with your fingers and balance the plane about 1/3 from the front of the wing. Add or subtract the stuff you put on the nose for balance.
Now chuck it!