Well assuming you have a portable grill of sorts.... you have options ranging from basic hamburgers and hotdogs upwards to include chicken, scallops, fish, veggie burgers, shish kebabs, BBQ ribs/pork/brisket etc. Prepare the stuff ahead of time (i.e. marinades or seasonings, items on the kebab sticks etc.) and bring it to your tailgate in plastic freezer bags or tupperware containers then just toss it on the grill. If your grill has a side burner then anything you might make on a stove top is pretty fair game as well.
You could also do simple prepare in advance appetizer type items like hummus w/ chips and veggies, cheese and crackers, chips and salsa, potato chips and onion dip, a deli platter w/ rolls, olive and pickle trays etc.