Having graduated from university a few years ago, I feel like visiting all my professors who influenced me for the better, and I'd feel awkward if I go empty-handed.
Just the fact that you are coming back is very nice and uncommon.<p>Unless you are trying to influence/bribe someone or you are from a culture of expensive gifts, gifts between professional friends are tokens of goodwill. A card or letter are both good, maybe some other small trinket but honestly a card is worth far more to me than some $20 pen in a gift box.