'Slip and Slide' - DS really enjoyed that around that age. Kept the boys busy the whole time.
We find that a pinyata is always popular. Best hint is that you can put the candy in sandwich bags before you stuff it, which then become the party favors!
"Hunt" were you set up clues that they have to figure out to get them to the next clue is good. Or plain old hunt - I've hidden 50 bounce balls in the yard - how many can you find.
Water balloon game - in teams of two, they toss back and forth, taking a step back for each succesful toss. Whoever ends up farthest apart is the winner.
You can always feed them, that takes up some time and provides natural pauses. Pizza first, and cake/ice cream later. OR cake/ice cream and Pizza during 'pick up' time so the parents can be encouraged to linger.
Plan something quite and easy for afterwards.
Prayers for a warm, fun party.
