Another teacher here, seconding and thirding gift certificates!
As a music teacher, I use the Borders gift certificates to buy CDs and books for my classroom. That way, all the children can benefit from the generosity of the parents.
You're so right - NO to mugs, jewelry (I'm very picky), food (esp. chocolate), knickknacks, and anything that says "teachers have class" or "I touch the future - I teach" (which always sounded slightly perverted to me).
As a piano player, I would love a gift certificate to any online sheet music website, or something like that. I could add to my own personal library of sheet music. Or again, a Borders-type of gift certificate.
Best!
Susan L.G.
Oh! Edited to add - yes! A note from a parent telling a teacher how much he/she is appreciated is PERFECT. On the occasion I would receive one, I would copy it, keep the original, and have the secretary put the copy in my school personnel folder and my folder in the Human Resources office. Better than any gift certificate is a heartfelt compliment to your child's favorite teacher!