I had my dd make a card. Actually, it was just a piece of paper thanking her for being dd's teacher, decorated by a drawing by dd of the teacher and children. Although I also attached a gift card to the local teacher shop, which the teacher did very much appreciate, the teacher was thrilled about the card. If it had just been the homemade card, without any monetary expense from me, she would have been almost as happy.
If you child cannot yet write, then how about brainstorming with your child to make a "top ten list" a la David Letterman about why your child loves your teacher. Then you could either have your child copy it, or you could print it out on regular paper, and have your child decorate it with crayon drawings.