A sincere thank-you note written by your child is the number one item I've heard teachers like to get--it's meaningful, heartfelt, and doesn't take up space in their home or need dusting.

My sister (teaches K-8 music) also likes disposable stuff--edible treats, poinsettias, etc.

Giftcards are also popular--Barnes and Noble and Target go over well, or giftcards to restaurants and coffeeshops if you know the teacher likes those places.