We just went through this on Friday. Yes, I was as nervous as you. Maybe more. I am the crazy old lady that cried when she dropped off her pup.
Shelby was hungry that night right after surgery but they told me to limit her food to a half portion. If your dog only eats dry, you can moisten it with warm water. I wouldn't give her any new food or treat b/c it might upset her stomach after going through surgery.
shelby is fine now. too fine. i want her to slow down until she completely heals!
good luck.