One of my favorite gifts from a family was a gold necklace with a heart. I wore that for many years, thinking of Danny when I would touch it. Cash is always welcomed. If she likes to have pedicures, buy her a gift certificate for that. Restaurant certificates are always nice, if she has a hobby (scrapbooking, crafts, etc) you could always get her a gift certificate for that. My favorites are the notes people send...I have saved every one of them.
You are very kind for thinking of her. Many times, people think that we are paid for taking care of their kids, that should be enough. What they don't realize is, if we did this simply for the money, most people couldn't afford what we are really worth. It truly is a labor of love, and quite possibly the best job ever!