I gave a Benchmade knife to my instructor, who seemed to appreciate it. I do think a gift of some kind is appropriate, if for no other reason than to keep instructors interested in instructing. None of them are doing it to get things, but showing gratitude in a concrete way does a lot to make the process worthwhile for the instructor.

I also agree with others about requiring people to get their own stuff. I instruct now (specifically because so many guys in the club went out of their way to help me) and look at the club trainer or my personal plane as only a way to get people interested. If I'm out and there are spectators, I usually offer to take someone up on the buddy box. It's good PR for the club and it shows people who are intersted that they can actually do this hobby if they are willing to make the investment.