Depending on what you are serving, you may want to bring a grill and fuel, coolers (one for drinks, one for food), platters/containers for food, plates, cups and utensils. You might also want to bring plastic wraps/foils, cleaning supplies like paper towels, wet wipes and trash bags. Be sure to pack clean plastic carry-away bags for leftovers, too. Don't forget the plastic totes with lids for hauling dirty dishes home.
If you want to create fresh blended cocktails or mocktails or serve heated dips from a slow cooker, check for a nearby power source to plug in small appliances.