Since you pay her, I don't think you need to leave a tip. However, you could bring her a little something from your travels.

The catsitting group I'm a member of catsit for free. I have done it twice, but one of them brought back some goodies from USA for both Fister and me. The point is to save to cost of a cat pension, and for the cat to be able to stay at home in the usual environment.