Nothing! The answer is nothing. Instead, if you are feeling it, write a card or a letter.
I always answer with this when the question comes up, but it really, really is true.
I’ve spent the morning packing and came across my file of letters and cards from former students. I keep every one. I’ve had a good old happy weep reading all their messages. I’ve been feeling a bit down on teaching recently, but these cards have reminded me of the difference I can make to people’s lives. And that is a huge privilege. So if you are ever agonising over what to buy, just grab a bit of paper and a pen and write a note.