For a basic kit for cuts, scrapes and punctures,
Antiseptic spray/gel, wound powder, salt, couple of bandages, cotton wool, gamgee, poultic, electrical tape for holding on bandages, scissors, safety pins, tweezers, thermometer, waterproof solid container to put it all in. Telephone number of vet.
Also well worth having few human bits and pieces in there as never have plasters when you need them.
Have I forgotten anything???