As a former teacher, I have to agree that big or small, the most valuable part of a token of appreciation is the note. I liked both notes from the kids and from the parents that included what they learned from being in my class, favorite memories, etc. If you can include a picture of the child with it that would make it even more special.
I enjoyed getting cookies, pumpkin bread, coffee, etc. Your kids might know that Mr. Smith LOVES Jelly Beans or rhubard pie, etc. so be sure to ask if they know of something he or she especially likes.
If they have a very limited classroom budget and most of their supplies have to come out of their own pocket, even a small gift certificate to a teacher supply store would be a huge blessing.
If you have those 10 minute chair massage places near you and they're open when the teacher could use it, a gift certificate to that would be wonderful. They might have gone up, but they used to be $10 for ten minutes here.
This is easier well in advance, but if you're handy, you might ask the teacher if there is anything that you could make for the classroom in appreciation for his/her hard work. This could be anything from fabric supply organizers to math manipulatives to puppets to really nice bulletin board sets. (You'd be surprised how long it takes to cut out and put together a single large bulletin board, especially in the early elementary grades when the kids can't help that much and the boards are expected to especially amazing. The premade ones at the teacher supply stores are ridiculously expensive. My first year, when I had nothing made from previous years, I had five that I had to change every month according to school policy.)
The vast majority of teachers enjoy books (or they would have chosen a different career) so a gift certificate to a bookstore or Amazon is usually appreciated. One to Barnes & Noble or Borders can also be used toward coffee or CDs.
I hope I'm not rambling too much. I've been staying up for the tornadoes and I'm quite sleepy. I hope this makes sense and helps some!
"...for the happy heart, life is a continual feast. Better to have little, with fear for the Lord, than to have great treasure and inner turmoil." Proverbs 15:15b-16 NLT
The pessimist sees difficulty in every opportunity. The optimist sees the opportunity in every difficulty.--Winston Churchill