If you can stand being around her (by your description, she sounds like a controling workaholic), offer to take her out to a nice dinner or lunch before or after class at a place near campus. If each of you chip in for her meal, it'll be only a few bucks on top of your own meal and she'll get a free dinner or lunch. Everybody likes food!
If you'd rather not spend any time with her outside of class, and she commutes a long way to get to class, you could each chip in toward a gift card for gasoline (since nobody likes paying for it these days).
If all else fails, I'd at least write her a meaningful thank-you note inside of a card you actually buy at the store (like Hallmark). Just remember it's the thought that counts, but you have to tell her in words how much you've really thought about it in order for her to actually care!
I personally wouldn't get her anything. I'd just thank her for her extra work, but mention that it really wasn't necessary and I that would've been happy with whatever grade I would've recieved for my own work. Maybe it's just me, but I get a little annoyed when people try to improve my work. How can you improve perfection?
