The Presidio of San Francisco is a 1500 acre park on the site of a former military base. There are innumerable places to meditate in this massive city park but one of the best is the San Francisco National Cemetery.
The cemetery lies in the heart of the Presidio and offers a number of excellent spots for the urban meditator. The lookout over the cemetery, the bay and the Golden Gate Bridge is the most dramatic spot but it is also the most highly trafficked. So the best bet for an solo session is to walk down into the cemetery and follow the road down to the bottom of the hill and perch yourself onto the low part of the wall along the Western edge of the cemetery, near the Southwest corner.
The only downside is the highway, though hidden from view, can be heard nearby. And of course it is San Francisco so dress warmly!