I give gift cards and I try to pick ones that I know fit the interests of the teacher. I usually have an inkling of what they'd like but I'll still ask the daycare center director or another teacher to make sure. In the past 5 years, I've picked gift cards to favorite restaurants,
Best Buy gift cards (I was told the teacher was wanting to buy a new DVD player), Old Navy (the teacher's favorite place to buy clothes), iTunes, AMC Theaters, and even Babies R Us (teacher was single mom with an infant). I usually make some homemade goodies to give along with the gift card. My chocolate and peanut butter fudge has been well-received.
I haven't always received a written thank you, but I've always been warmly and sincerely thanked for gifts.