This is from a site for older students but this idea we could do as a class.
Zip line racers. I think I would make the frame for the racer and the students could experiment with different sizes of rubberbands.
Marble roller coaster
Very cool water wheel. I might modify it by using bottle caps instead of egg cartons so it lasts longer. We could build both and discuss the pros and cons of the different designs.
Here is another water wheel design
These look like so much fun!
Boat designs. Again I would make different designs (since it involves using an exacto knife or box cutter) and have the students test them.
This might be an interesting project to do. We would make a model of our school using cardboard.