Stick a galvanized nail into an apple or a potato. Stick a bare copper wire in close to it, but not touching. How much voltage does your homemade battery produce? Does aluminum work better than the zinc on the galvanized nail? How about using Coca Cola instead of the vegetable? Or just placing the electrodes on your tongue?
Does a solar cell work better than an LED as a light sensor?
What do you get when you use a blue LED instead of a red one? (White LEDs are actually blue LEDs with a yellow phosphor on the top, so you can use a white one if you don't have blue).
With the blue or white LED, what happens if you shine only red light (like from a red LED or a red laser) onto it? Do you get a different result than when you illuminated it with white light? Does a red LED do the same thing?