A friend of mine was given a watch, which was pretty cool.
I've been given a few bottles of whisky or wine, and one bottle of rum. I think my students knew me well...
Even a box of chocolates is well received and shared around the instructor room. Depends what sort of relationship you have, I saw a few of my students outside of the flying school setting, and I am still in touch with them long after they completed their PPL and I moved on from that school. There are others I wouldn't want to see socially at all.