M.L.
There are a few parks in Petaluma that are good for parties. For one your size I would try Mc Near park on the west side, its on G st on the west side. There is also Lucchesi park On Mc Dowell Blvd, or Leghorn Park next to the G & G shopping center. Check with the city though because you may need a deposit for a group as big as yours. Hope thats a little helpful for you, and good luck.