-fruit (platter, salad, kabob, chilled soup)
-'cheesecake bites' (we make them in mini-muffin pans: a nilla wafer, instant cheesecake, one cherry each)
-cookies (though then you might have to offer milk...)
-bread/muffins (pumpkin, banana, zuchini, etc.)
-ice cream or popsicles--if you can keep them cold
-rootbeer floats--again, if you can keep it cold, and have someone to make them on the spot
It's probably easiest to serve a baked desert that works well at just about any temperature, this way you can just set them out, provide covers to keep bugs off, and not worry.
Personally, I'd probably chose a fruit option and then either scones or baklava.