is this an indoor or outdoor party?
will there be music?
It would be nice if it was both.
Start off in the early afternoon with a picnic spread like you mentioned. Volleyball, cornhole, baseball and a touch football game. (Someone already mentioned they are bringing a football team.)
After the fun in the sun, clean up for dinner, dancing and drinks indoors. Then if anyone is still up for it smores over a camp fire afterwards, passing a bottle of good whiskey or homemade moonshine, until sunrise.