Another vote for a running club, esp. if you have any parents who are marathoners or serious runners and have flexibility in their schedule and can lead the club.
We had one in the fall - two marathoner moms led it for 2nd - 5th graders at our ES. They did have a minimum grade requirement of 2nd grade. It was 1-2 days a week total. (Originally was going to be 2 days, but due to high interest, they split the schedule for 1 day for 2nd/3rd graders and 1 day for 4th/5th graders so more kids could participate.) Cost was fairly minimal. The running club also provided a healthy snack for kids.