I (still) don't have any friends, so on 1st year of high school I tried to be as interesting as possible. I kept bringing some electronics, but most interesting was fluorescent light bulb and Tesla Coil. OK, OK, it's not a Tesla Coil but a slayer exciter circuit, but it's close enough.
Other stuff I can remember is: USB buck/boost converter (basically a low power variable power supply), USB alligator clips, USB tester, smart load tester (FX-35), battery charge tester, hand crank radio/flashlight combo, low power FM radio transmitter, bootleg NES game console with composite video output, blank DVD, universal TV remote, 5 phones at once and other stuff.
Although, you could say the most unexpected was the HP PSC 1315 printer. One teacher told me I can take it home, and it fit precisely into my backpack.

Nowdays, 2 years later, the best you could find is RTL-SDR and the extendable dipole antenna.