I would put maybe canned food(if you feed it)or a small supply of dry, and toys! Maybe a certificate(that you get for free) to a petstore or something...Definitely toys...anything else you can think of? Hm..maybe just lots of treats...:)
I just picked up a kitten Friday and the breeder included a sample of the litter she uses and a sample of the dry food. VERY helpful so the kitty doesn't have to get used to new brands. She also had a package of info regarding the vet care the kitty already had. Other than that, all I needed was advice on any habits the kitty already has.
every kitten i send home has a hand made cat toy i make myself, from old fabric scraps or old clothes. and i use anything from reflector yard sticks to bamboo to cut up fishing poles to tie and rubberband the fabric strips to and the cats love it and the new families appreciate the homeade effort. also being a part of the royal canine breeders program gives gift bags to pass out to new families as well with a book and bag of food. it works for us. oh and a slideshow of the litter growing up from birth