Building ancient Greek computers out of legos. That's pretty cool. Now somebody needs to build one of those ancient Greek robots out of them. Yes, the ancient Greeks also invented robots. But, you could do that rather nicely. Have a play put on by replica Greek robots (without cheating with electric obviously, powered by sand the traditional way).
What other cool things could be built? There's some movement to create a complete Babbage computer, there should also be a sub project to make one out of legos too.