I had a guy get me a copy of the movie "One Six Right" on DVD as a present for doing his instrument training. I thought that was pretty unique and cool.
Along the same lines, I always enjoy good aviation books. "North Star Over My Shoulder," "Fate is the Hunter," "The Right Stuff," or any other number of legendary aviation texts. Or, if you know he has interests outside of aviation, nothing wrong with getting him a book that relates to something else.
A pocket knife/multi-tool is always nice, too.
And ditto the ideas about a gift certificate to a restaurant and/or just taking him out to dinner. Every now and then a student will take me out to eat and I always thoroughly appreciate that sort of thing.
Alcohol is an option, but make sure your instructor could use it. I'm in the minority of pilots who don't drink, so any gift would be politely accepted and dumped down the kitchen sink later.
You know, that just made me think though, a really nice coffee tumbler could make a good gift. I got one for Christmas a few years ago and it's proved extremely handy. One of my old instructors would always drink coffee out of a styrofoam cup and leave the empty cup in the back of my plane when he left. Kinda pissed me off at the time and I wished he'd have had a tumbler instead of always leaving his trash for me to clean up
