My family has been having our reunion at the local park for the last 25 years. Before that, it was held at various family farms. The family has been having reunions for 102 years starting with the first reunion of a family of 12 siblings! It is a picnic style gathering with several hundred people in attendance. Each sub-family sits together and arranges their own picnic for that group. A hat is passed around for donations to pay for the musical entertainment (this is usually a blue-grass band) and pavilion rentals. Some sub-families rent a tent. There is also a 50/50 drawing to help raise some money. There is one father/daughter team that arranges everything and they are in contact with a person from each sub-family. My family brings a large photo of the "original 12" family members from which we are all descended and displays it on a table. Everyone contributes to a smaller family photo album which gets passed around as well. The father/daughter team arranges the entertainment, pavilion rental, park fees, children's entertainment, and even gives out some little prizes which include 1) traveled the furthest, 2) oldest in attendance, 3) youngest in attendance.
Having it at the park makes it easy to get to, ensures entertainment for the kids (playgrounds, ball fields/courts), and shelter in case it rains. The fees are small and there are also bathrooms and lots of parking. The park location also takes the burden away from one person having to host it.
Each year we take a photo of our entire sub-family and add it to the photobook that gets passed around.