There is always some fun trying to get them to do certain races.... if you maybe mark out a 50m track (if outdoors) or from one end of the room and back (if indoors), and set them into teams. Blow up a balloon for each team. One at a time they have to run to the end of the track and back with the balloon either tucked between their knees, which leads to some interesting running (then no-one can be mean to anyone else, because they all look silly) - or under their chin (which is equally silly because they can't see where they're going properly). If tucked under their chins they have to pass it to the next person in the team from under chin to under chin without using their hands. When the balloon is dropped, there is a penalty, say running the track once more for the team. That way they have to learn to either co-operate to get the ballon passed between people - or find a creative way of learning to cheat together as a team.
Three legged races are good too, because then they only have to learn to co-operate with one other team member, rather than a group of maybe 5 or 6. All you need is a length of cord to tie the left leg and right leg of two kids together, and then they have to figure out a way to run in-step (just on opposite feet) in order to go anywhere. If one falls, they both fall. You'd be amazed at the incentive they can get from not wanting to be the only pair who fall over!