Point Isabel is a great off-leash dog park. Don't go if you expect to have a quite picnic lunch or sunset on a lawn....unless you want to share with lots of dogs. Don't wear white. Dogs have access to swim in SF Bay. Don't ruin it - Clean-up poop, and no digging. There is a cafe = food and drinks. Drought and budget cuts have turned off the water to irrigate the lawns = it's pretty much dirt now.
Great Bay Trail Access - is separated from dog park by fence = dogs on leash. ADA, paved, very flat, and runs along the bay.