I tend to use the huge gold/red or silver carriers from The Card Factory for 99p for the kitten packs which are usually made up of

Assorted mice (rainbow/leather/catnip/honeysuckle/felt/fur)
Assorted balls (crunchie type/with bells/aromatherapy/ping pong and so on)
Spiders
Matching bowls and blankets (blue for boys/pink for girls).
Knotties
Usually put a harness/lead in too
Yeoww Catnip Banana or Rainbow or similar
At least two different wand teasers
Special breeder sample packs of Craze treats (eBay.co.uk Shop - Pet Food: Craze Cat Treats: CAT FOOD, CAT TREATS, THRIVE - you can e-mail them and ask them to do a special pack)
Grooming brush/comb
Cat cube
Pack of drinking straws (they love to play with these)
RC, JWB and Hills Dry Kitten
2 boxes of wet kitten food Whiskas/Felix
Couple of tins of Applaws Kitten
Tin of boneless sardines
Litter
Paid membership for first year to breed club (our club offers breeders a 50%discount for introducing their kitten owners to the club)
Pedigree/Pink Slip/Vaccination Card
Diet Sheet/Sheet about kitten/s likes/dislikes etc
General Kitten Info - Poisonous Plants/hazards etc.
Contact numbers
DVD with pix/vids of kittens from birth to day of leaving

I do tend to pick up the packs of mice and balls etc a couple at a time throughout the year if I am at a show and put them away so it is not such a huge layout.

I was thinking next time around I may get a large box and just get some wrapping paper with kits on and just back it in that.