How about learning violin or piano? Cello is tuned in 4ths, why not give that a go.
Can you play tunes just with pinch harmonics? That'd be cool. maybe you could try two guitars at once (Michael Angelo style) and transcribe chopin and rachmaninov until everyone worships you.
Failing that learn how to play guitar whillst locked in a box under 12 feet of water with your hands tied in chains.
This was suggested to me by my neighbours (lol!).