A gift sounds like an overkill, regardless of what you may have heard I've read/been told numerous times by adcoms that even though sending a card is a nice gesture it's in no way going to influence your chances of getting in. If they want you they want you, if not a gift isn't going to change their minds. A gift just sounds like too much, IMHO.
I think it's okay to send a small gift with your thank you card for LoRs. I gave a small, restaurant gift card to the two recommender's I was close to. For the third recommender, I just sent a thank you card.
Oh, my bad. I guess I misread your post. In that case getting a gift is totally appropriate.
I went to See's candies and got them chocolates. They seemed to be happy. My bf's PI who wrote him an LOR is a big fan of herbal teas, so my bf went to Lupicia and got him a box with variety of herbal teas. Find out what they like and enjoy and go from there.
A professor of mine still had not written my letter for a supplemental application after I had given her more than a month's notice. It was getting EXTREMELY close to the due date with no replies to my phone calls or emails, so I basically had to ask another professor last-minute (a few days before the letters were due). Not only was he happy to do it, but he got it done the day I sent him the forms. He was my favorite undergrad teacher, and I should have asked him to begin with...
Anyway, he's a really big Simpson's fan, so I'm thinking to get him a Homer mug. Something simple, yet useful. =P
...And if you don't know what they like, gift cards all the way!