I don't think you're expected to give anything. A thank you card will be more than sufficient especially as they've not been overly generous.
I've witnessed four or five poeople retire at my place of work over the last couple of years - all had been with us as long or longer than you. We collected weel and gave them a good send of with champagne and nibbles, plus a gift etc. But they didn't give anything in return. Just a card a few days later to say how much they'd appeciated thier time here and the generosity of eveyone.
Enjoy your new founded freedom!