No need to get fancy, just make sure you have enough. If you're expecting a crowd, a nice cold keg is great. If you don't want to deal with a keg, buy six-packs or cases and keep them cold in a big cooler. If it' s a chilly afternoon, bring thermoses of hot cocoa. Especially if you have a keg, the more cups, the better.
Folks get pretty psyched when there's plenty of food around.
Hot dogs and hamburgers fired up on a portable grill are great. Don't forget buns and condiments. If you don't have a grill, try a selection of cold cuts and sandwich bread or get one of those six-foot subs and let everyone cut off a section. And finally, bring something to satisfy your sweet tooth. Brownies and cookies are great no-mess options.