GaryL,
This question comes up every now and then. I can't speak for all instructors, but as for myself I don't need anything. My reward comes when I see you solo and become an active part of the flying field. If an instructor wanted something for his efforts he would have charged you a fee up front. If you feel that you must do something for him then do something that has the greatest impact. I had one student buy me a Chili's gift card. Or maybe a gift certificate at the LHS. But I doubt that he expects anything.

Ken