I agree with the others, a nice thank you note is the best. We know that you work hard and put so much into teaching our children and we want to help and support you.
My dd's K teacher did get a small wooden box from a craft store and painted it very simply and had my dd make a special picture to put into it and then the thank you note from the teacher went in there. I have it sitting on my dresser still to store "treasures" from DD in. Something very simple and sweet like that, or a simple (handcrafted?) picture frame with a photo of you and the child, or the whole class, or thank you notes from the rest of the class to the room mom (great educational activity!) would all be appreciated.