Prime [I mean prime prime] location in San Francisco is Union Square. it is named after the the big pro Union rallies during the Civil War of 1861-5, very important events bringing California in the war on the side of the Union.
Square is in many ways in the Hearth of the city. It is marking the centre of the shopping district, with Macy's, Saks Fifth Avenue, Levy's, Louis Vutton and many other expensive shops. It is starting point for exploring Theatre District, in close proximity is complex of Jerba Buena Gardens, SF Museum of Modern Art and along one of its side streets world famous cable car is running since 1873.
On the square itself, stage and of course enough space for spectators is providing occasional performances and fulfilling its reputation as an ceremonial square. Nice caffe with decent menu is where pleasant and discreet people watching is satisfied.