The San Francisco Bay Bridge is made up of two spans, its western section reaching from San Francisco to Treasure Island and the eastern segment running from Treasure Island to Oakland. Eastbound lanes travel on the lower deck, and you can see little from them. The view going west is much nicer.
There are no pedestrian walkways or vista points on the bridge, but you can drive on the bridge and stop midway at Treasure Island for a nice view of the it and the San Francisco skyline.