Lots of towels and disposable bed sheets - they make these for humans and you can line the whelping box with layers of them and just throw away and replace during whelping as needed.
Scales for weighing newborn puppies.
ID collars.
Calcium - to give AFTER the first puppy is born onwards.
Milk replacer just in case, and bottles.
Sterile scissors if you need to cut any cords (you shouldn't need to, but in case)...
A bulb syringe for cleaning out mouths of mucous.
And probably lots more things I can't remember right now...