It's gracious to offer something and since you can't buy them a drink in FC, a twenty or a gift card (Starbucks is in most airports) would be a nice gesture. Taking a food gift from a stranger can be an issue unless it's tamperproof, although good chocolates could work if the package is sealed.

More importantly, ask nicely and say thank you regardless of the answer.

Aisle for aisle should be easy, but try to board early and ask the "victim" before he/she gets settled.