have a PE related activity or health related if you can, a stock of worksheets/books/sports biographies/newspapers sports reports with comprehension sheets etc ready to use...make it relevant and interesting though
forgotten pe kit may be the sign of something else too, maybe they dont want to change in front of others....signs of abuse or eating disorders/bullying/maybe parents cant afford the kit/trainers
some schools keep a stock of old kit, i know lots wont use it, maybe if it was kept clean and available to use, dont know if there is access to a washing machine
discipline...dont know what is allowed in your school..detentions will have to be arranged by teachers, cleaning out the sports cupbard is boring and mindless

a hmework task to be returned the next day