keep paper plates in stock. depending on how long people stay, you might run out of your normal plates.
make sure you have extra aa batteries for the game remotes i talked about above and for cameras.
create a "lost and found" box. we always end up with extra clothes, socks, shoes and toys. if you have a "lost and found" box, people can look through it each time they come back and get their stuff.
if you have the space, keep extra jackets around just in case someone forgot theirs. even when my kids grow out of their jackets, we keep a couple extra. that way, if the kids get too crazy 