Definitely don't need anymore mugs or junk-food. I always liked NICE paper for notes home or to write whatever on. I got sick of off-brand postits that the school provided.
I never got a gift card, but would have LOVED one. Maybe for a spa/mani/pedi ?? For a favorite restaurant? Clothing store?
My room was decorated with frogs, so I often got more frog stuff. Fine - it added to my room, but not anything I dreamed of.
You probably don't have the teacher's home address, but a magazine subscription is something we don't often buy for ourselves. If she has kids, maybe a Parenting magazine. Readers Digest is nice for all ages.
If she has kids, maybe get something for her to do with them during the holiday break. A craft, some books to read with them . . . .A friend just recently got my son a gift certificate to a local kiddie park. Cost only $12 and we had a blast. Maybe the nearest aquarium or zoo??
I wish I had the perfect answer. I can't think of any gift that I ever liked. I once got a half-bottle of used perfume from a student. Smelled awful. But he was so happy to give it to me. He loved that I sprayed it on my wrists and smiled at him.