When I was teaching, I had a parent that brought me in fresh blueberry muffins every month or so. It was so nice because there were often times that I was so rushed to get things ready for my classroom that I didn't have much time to stop and eat breakfast.
Gift cards to places like Starbucks or anywhere you find out that the teacher likes to go are always a huge hit too. She may not be a coffee drinker, but may like soft drinks - then Sonic may be a good place. ($5 at sonic buys about 4 drinks).
Another thing you could do is find out her favorites - drinks, foods, candy, snacks, etc... Surprise her at the end of a hot day with a huge drink of her choice and maybe a snack or candy bar. We had wonderful parents where I taught, and they always came up with very creative ways to make us feel special.
Whatever you chose to do, I am sure that they will be very happy to know that you took a little time to think of them.