My vet did this for us, and I think doing the same for them would mean the most. Donate to a pet rescue (Lost Dog and Cat Rescue Foundation, Humane Society, APSCA, etc), and there is a spot to indicate that the donation is in memory of (your dog), and there is a spot to indicate that you gave a donation in your pet's name on behalf of the veterinarian's office. Then the person/business will receive a card.
It meant so much to me to receive a card that read, "McLean Animal Hospital has donated $________ to (whatever rescue foundation) in loving memory of Abby." It made me feel that Abby's life had been honored and that she was helping others.