You can find no better place than a picnic area or campground. There is no need to rent an expensive dining area or hall for your family reunion.
Nobody has to be stuck with having the reunion at their home every year.
A nice public area will provide you with a perfect economic solution. Many picnic areas provide large group sites where your family can gather comfortably. Choosing a good picnic area should be done a few months in advance. Assure that the parking at the area of your choice will be adequate. Many parks offer over-flow parking.