I don't have a travel agent (we do all our own bookings via the internet) but I did give someone a cruise-related gift once. Last year I won a cruise in a contest in a mall. The winning ticket came from a store I frequent to have jigsaw puzzles laminated. The owner expressed interest in a puzzle I did of the Caribbean (bought in St. Thomas), so on the cruise I won, I bought another puzzle in St. Thomas and gave it to her as a thank-you.
She loved it!
P.S. I agree with Mike, the best "thank-you" you can give is a referral!