I can only reply as a teacher... THIS teacher would appreciate a six-pack of good beer and a new copy of the fifth Harry Potter #5 (mine has mysteriously gone missing). Well, not the beer so much this year, since I'm all pregnant and stuff (and I'm joking anyway since bringing beer to school is inappropriate in so many ways...). But my point is... find something that matches the teacher's interests.
It's like buying a gift for anyone else, really. A "good idea" for one person is totally different than a "good idea" for the teacher in the next classroom. Does she come in every morning with a big mug of coffee? Maybe a bag of really good whole bean coffee or a gift certificate/card to a local coffee shop. Does she knit in her free time? Run? Sew? Have dogs? Like Thai food? Homemade, gift card, food, etc... they're all equally great options if they match the personality/interests of the person you're buying for.
