Idea 1 - Game

Look at using java or blender, to make a simple game that would require the user to slot various shapes in various sockets

cost  - 1 - Nothing

time - 2 - Maybe 2-3 days.

Pros - costs nothing, no new skills need, would be one of the more interesting ideas for me to do

Cons - Time could get away from me, wouldn't be learning new skills just improving others

Idea 2 - Toy

Very similar to the game only will be much simpler and be a wooden frame that blocks of different shapes can slot in and out of.

cost - $50-100 + would need to borrow tools, and steal some wood stain(very expensive and need different colours but small amounts).

time - 1 - Could probably get this done in a day, other than waiting for glue and varnish to dry

Pros - Would be the most fun to make, having something physical is appealing.

Cons - Might be too simple, expensive(car still needs a front bumper!).

Idea 3 - Animation

essentially idea one but playing itself.

cost - Nothing

time - 3 - Mare time than I think I have -Would need to learn how to use blender or 3DSM :s

Pros - costs nothing, would learn a lot.

Cons - Would take a very long time, essentially the same as the game.