Since the grill is already on, maybe do up some baked potatoes in foil? How about soaking the corn ears in water and putting them on the grill to cook? For a summer party, I also use my big turkey roasting pan and fill it with watermelon slices and put the top on and have it in the fridge. I bring out the whole pan to the picnic table and you have the top handy to keep bug off it. If it's a fancier party, I do up the fruit bouquets with chunks of fruit on bamboo skewers in weighted containers.