Our school has an organized "walking club" sponsored by the PTA. They track mileage for each kid (and give out little plastic prizes when the kids reach milestone numbers of miles). It would be some work to set up, but it was really key for my DS9 (2E Asperger's) who had similar issues of not being able to occupy himself at recess in earlier grades.

He still likes the walking club; it appeals to his number sense and he likes to rack up the miles.

DeeDee