Coils and magnets
The idea of this one is that students come up with research questions related to electromagnetism but they could also have ones to do with mechanics if the coils are used as springs rather than solenoids. I'll stick to electromagnetism here though. In addition some assorted coils and a load of wire you could also supply some magnets. I bought some Neodymium ones from ebay that are very strong, when they arrived late...