Here's everything I have learned to play because it was easy as shit:
Flute (hardest thing is getting the mouth placement down. Once you have that you're on easy street; and I think this is probably true of all woodwinds and brass instruments)
Mouth harp (boingy boingy boingy)
Ocarina
Harmonica
Theramin (pretend you're a wizard!)
Tambourine
Taiko drums
Bongos
Xylophone
Cymbals
Cannon
The only instrument I have learned that was difficult and took years to get even semi okay at was piano. And I mostly attribute that to my small hands, because I can play so much better on a MicroKORG than a full size keyboard.