My sister pet sits occasionally. Of course she loves to get cash but she is a Starbucks fanatic and prefers gift cards to Starbucks. Do you know you sitter well enough to know her/his favorite things? A gift card to a place they love is always nice! I love receiving movie tickets as thank you gifts. Depending on where you are going, a trinket from your trip might be nice, too.
If you are creative at all, you might want to make her a card "from the cats". Use handwriting and language that sounds and looks like a cat might have done it. (I try to write like a little kid- a couple letters backwards, no upper case, etc.) They are always cute!
Hope you have a great vacation!