S.B.
You can never go wrong with a card, but you might also ask if they have a fund or organization that they would like you to donate to such as spay/neuter clinics, the SPCA, programs to re-home abused or displaced animals.
I bet they could give you lots of ideas.
Yes, a card, some home made goodies...but I would go beyond that in helping other animals to receive such wonderful care.
I'm sorry for the loss of your pet, but maybe you can start a fund in her name or make donations in her name. It's a wonderful way to memorialize the years of happiness she gave to your family.
Best wishes.