With budgets tight these days, a park is a wonderful venue for a family reunion. You may be able to reserve or rent a covered pavilion, although some parks have a first come, first served policy. After the grub and socializing, there's plenty of space for kids to run around and lots of play options to ward off boredom while the old folks sit around and reminisce.
Does your family need more than one day together? Many state and national parks have lodges and camping accommodations, so you can extend your family reunion beyond a single afternoon.