Advice from a preschool teacher-- GC to Target or the like are ALWAYS great, stuff for school use that we end up buying purselves (paper, crayons, glue, etc.) also good. Food and a nice note go very far.
Avoid school knick-knacks... I really don't want a million apple trinkets (but then again, check out my little siggie QT! Ha ha!) The notes, I really do keep in a "happy file" so if I'm having a tough day I have somewhere to refer to so I remember why I do this. HTH!