Have to agree with everyone on this as a former teacher...gift cards and amount is not important. Five or ten dollars is fine. Starbucks...Jumba Juice, Target, bookstore. And even more important is a card from you and your son saying specifically why you appreciate what they have done. I still have some of the cards parents gave me.
Another nice thing is supplies for the classroom. I often spent hundreds of dollars each year on supplies. If you could get her something for the next school year, I'm sure she would appreciate it.
No to candy, cookies, teacher mugs and desk stuff, frames (unless a picture drawn by your child is in it).....anything that will just collect dust. When I stopped teaching when I had my kids I ended up throwing away so many things students have given me...I had no place to put them and didn't want to bring them into my house.