I'm going to list ideas of things I would be interested in making or have already made. Feel free to take any of these and make them for yourself or for the maker fair. The idea is to share and get people thinking. If you have any ideas list them too.
1.) Open source off road go-kart with plans. May already exist I am not sure but I want to do my own. By off road I mean very strong, with a roll cage, and a heavy suspension.
2.) Bike powered utilities. A bike powered water pump for crops or showering, bike powered generator, etc.
3.) Water powered generator.
4.) Walking machine using hydraulics or pneumatics.
5.) Simple home made boat. Could also add a home made motor.
6.) Gas powered wheelchair.
7.) Log splitter that doesn't use a hydraulic ram. This I don't have a clear concept of cause there's many different ways to do it. Here's some ideas.
8.) Paddle wheel boat. My idea is sort of like the one in the link but more refined and with a ride on mower driving large paddles off of the sides.