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Take an empty plastic water bottle (note that all water bottles are made from a very thin plastic) and fill it with very hot water from the kitchen tap. Make sure the water is hot and let the hot water flow over the outside surface of the bottle also.

Then quickly pour the water out and put the cap back on the bottle very tight.

Lay the bottle on it's side on the kitchen counter, or stand it up, and listen to the noises it makes and watch it collapse as the hot air and moisture in the bottle cool and condense to reduce the pressure in the bottle to below atmospheric.

If you boil water in a pan on the stove and then pour it into a plastic two liter bottle it gives even a better effect.

maybe not science fair material, but this is fun! we did it in school once..get a 1.5 or 2L bottle of soda, anything will do but there is a difference between diet & regular soda, i cant remember which works better but both work. Leave it out in the sun for few hours, then unscrew the lid and cover it quickly with a piece of card, to prevent the gas from escaping. Hold a test tube filled with Mentos sweets directly over the bottle opening and slide away the card to allow the sweets to fall into the bottle. Result? A 10 foot soda fountain! This is fun, especially since we weren't expecting it. Needless to say we got soaked!! (",)

mix vinegar & baking soda make ooblick --- mix water & corn meal! (look it up!) (cause it may be something other than cornmeal) add some food coloring Its amazing! Its hard when you touch it but it melts in your hand at the same time.

Why don't you buil a Heron fountain? It is fun an easy to make, and helps learn something.

http://en.wikipedia.org/wiki/Heron's_fountain

build a small solar powered generator to run a standard light bulb.

how about the diet coke + mentos rocket? Look it up on youtube