Crissy Field, part of the Golden Gate National Parks, is a recently restored stretch of beach and parkland next to the bay. It's a very popular place to take dogs for a good swim. Currently there is a big debate about dogs in the national parks - see Dog Management at the GGNRA site and SF Dog.
If the weather is vaguely nice then the beaches at Chrissy Field get pretty packed. We go to Fort Funston most weekends, and save Chrissy Field for when the sea is too rough or (like today) when we just can't face climbing back up the sand ladder.
Hike starts at: 37.806083, -122.450172.