J.G.
Try the old fashioned games. Keep the kids moving. It will wear them out and make for oh-so-happy parents!
Bean bag toss
Potato sack races
water ballon races
3-legged races
musical chairs
hard-boiled egg on a spoon race
pin the tail on something (or for example is the theme is 'cars'..pin the wheel on the car)
I saw a lady in the park this weekend directing these games for her 5/6 year old son's birthday.
Here's a few sites:
www.best-kids-games-online.com/outdoor-games-for-children...
www.fungameskidsplay.com/outdoorgames.htm
childparenting.about.com/cs/kidsgames/a/outdoorgames.htm