Low-noise circuits is a good idea, for example bipolar transistor amplifiers. There are a lot of potential parts to test, each part is very cheap, but it would be more economical to buy them in quantity and send everyone 3 pieces of each. One could setup a simple circuit that connects to an audio card to make a spectrum analyzer and measure the noise.
One reason I have bought kits is to assemble them with kids. Here it is more for fun: Halloween, Christmas decorations, motion activation, flashing lights, etc. There are some from Elenco, but with a little imagination one can think of more permutations.