A guitar can go a long way at hostels and beach parties, for sure. I even met a guy in Beijing who was staying and eating for free at his hostel because he would play a couple of hours for the dinner crowd every night.
Learn 1 magic trick and work on only that trick until you perfect it. Make sure the only prop you need is something common like a cigarette, coin, whatever...sounds funny, but it works unbelievably well for breaking the ice in group scenarios.
As for paying back a couchsurfer...just be a good guest! Don't just 'use' your host for a place to crash. Hang out, chat, get to know them, become friends. That's why they are hosting in the first place!