Speaking of stories, how about having some of the older family members tell something about their parents or ancestors. This is always fun and very interesting. It will more than likely be a hot day, and little ones love to 'buy' things. Only raise the prices just enough to cover your cost. If you are having the reunion at someone's home, fill an old bathtub or something similar with sawdust and bury some pennies, nickels, dimes and maybe a few quarters in it.
Little ones love searching for money and spend hours there.
You will want to put an age limit, maybe have two for different ages, or the older ones will crowd out the younger ones. If it is an especially warm day, get a big peice of plastic and lay it out on the lawn, room permitting. Place a hose that is running at the top and let the kids run and slide in their swimsuits. The grownups have just as much fun slipping and sliding as the youngsters do. Hope these ideas help.