Alot of food companies have breeders schemes that provide free kitten packs that include food, bowls toys etc they all have different rules but mostly its that you have to buy so much of their food. RC, James Wellbeloved, Hills, Techni cal, MPM products (Applaws & Worlds best cat litter supplier) all have breeders schemes. There may well be others. For additional toys, I just keep my eyes out for packs of mice and balls, tescos sometimes have little packs of various toys that are ideal.
The only other thing is the paper work, and theres a bare minimum you have to provide as per whatever registering body you register with. What you provide over and above that is completely at your discretion. Most pet buyers are only really interested in their new kitten and don't take much notice of the paper work.
I give mine a folder which contains :
- About your kitten - includes details of when where they were born, how much they weighed, the rest of the litter, pics of their Mum and Dad, what they like and dislike, and pics of them growing up
- Diet and Care sheets - keeps getting longer

- Copy of the contract
- Pedigree
- GCCF Pink Slip
- Insurance Cert
- Microchip form
- Vaccination card
- Receipt
think thats it, then that goes in a gift bag with any extra toys, packets of food they enjoy and a kitten pack from technical because they give a pack of litter I use - Oko Plus.
The most important part though is a lovely healthy, well grown, well socialised baby who is ready to take on the world, and a life time of support if needed from his/her breeder.