Over the course of my 3-year, on-again off-again training, I tried to give the then-current CFI a Christmas gift of cash or a cash equivalent (gift certificate, paid time on the books of the FBO) equal to one hour of the total rental fee I was then paying -- wet Hobbs plus their normal CFI rate. I figured their 'value' was at least equivalent to one extra hour a year. The really good one got a gift equivalent to an average lesson length. The younger CFIs working on ratings of their own, like multi or commercial, were always most appreciative of any time they didn't have to buy themselves. The older one just liked the cash. I know this is a post-Christmas post, but might be useful as an idea for future "occasions."
Elizabeth
PS Line guys who really helped during the year got a 20. FBO office staff who might have gone out of their way also got a 20.