Your dog needs to reliably come when called to be a responsible dog park visitor, especially if the play area is only partially fenced. If your tail-wagging companion fits the bill, you're a great candidate for Mill Valley Dog Run , (a.k.a. Mill Valley Dog Park), part of Bayfront Park and Bothin Marsh Open Space Preserve. A three-acre field next to an estuary is designated dog-friendly; it features agility areas as well as grassy space and a splash-happy area on Richardson Bay.
From mid-March through mid-October (a special event is planned for Dec. 31), park users gather for a "Fridays in the Park" yappy hour to share refreshments, talk story and get to know each other beyond just their dogs' names. It's very Marin...and very fun to experience even if you don't live here.