I know gift cards have been mentioned but thought I would add a few more specific gift card ideas:
Gas gift cards (who wouldn't like this nowadays)
Grocery gift cards
Garden Center gift card
Coffee Gift card (someone already mentioned this... but only get it if you know the teacher really likes coffee otherwise she will never use it)
Put them in a handmade card that your child helps you make to personalize it a bit more.
One of my mom's favorite gifts has been the handmade stepping stones. The parents from her preschool class all got together to pitch in to make them. They used glass and stones in cement. My mom loves gardening so if you know of a hobby that the teacher enjoys do something along those lines... that makes it very personal.