The object of this is to tell everyone one thing you do, one idiosyncrasy, that you will never give up. Whether they offer money, sex, anything, you can't say with certainty that even though you might quit now, you won't pick it up again later. If they threaten to end your relationship for it, you still won't quit. After all, it's a part of you.
This can be anything from big to small. It doesn't matter if they like it or not, just pick something that you don't feel you should ever have to quit. I'll go first:
I frequently walk/run to and from places. Sometimes that place is miles away, and forces me to travel on foot next to busy streets. She hates this, because it's dangerous. I like it, because I get to see things better than just passing it in a car.