A really fun game is "Dodge Ball"; but use sponge balls rather than soccer balls! They are easier to hold and will not hurt the players when they are struck by one.
Make sure you have enough balls to hand, and vary it so that it is played in teams, as well as the usual "Tag" way.
"Tag" is one of my daughters' favorites.
You can play it the usual way (with one person passing the tag to another), or you can play it so that one person is the tagger and if they tag someone else more than three times, they swap places (or maybe become an additional tagger).
"Stuck In The Mud" is another variation where, if someone is tagged, they have to stand still until a member of their team runs under their arms to 'untag' then.
"Hide and Seek" is also good fun.
By playing it outside, there is much more scope to be creative.
However, make sure there are enough adult supervisors and that the children know where they can and cannot go.
"Scavenger Hunts" can be fun, though it is a better idea to have these at home as you do not want the children wandering off too far, and strangers may help themselves to the prizes without you knowing, thereby wasting your efforts.
Visiting a local swimming pool is also fun.
Again, having enough adult supervisors is vitally important, but there are plenty of games involving races and pretending to be various animals that can be played in a pool environment.
If you go in a group, you might be able to negotiate a discount.
Using Hula Hoops is also a great way to have some fun.
Be sure to have your camera handy! You could get the players to compete and see who can keep their hula spinning the longest.
Healthy prizes to the winner! All of these outdoor games can be played in teams or by individuals, but it is always a good idea to include a variety to ensure everyone can be involved and enjoys themselves.