Downtown San Francisco has many public spaces known as POPOS: Privately Owned Public Occupied Space. These are located in office buildings and hotels, and are available for the public to enjoy during operating hours, which vary from location. The the public rooftop of 343 Sansome St. is one of the best hidden gems in the city, but few people know about it.
To get up there, go to the 15th floor. You may have to check in with the bored security guard and tell him where you want to go. The other floors are only available by touching a keycard to a number pad, but the 15th floor is open to everyone.
It is open during business hours, so figure 8-6 weekdays. 9-5 just to be sure.
The are nice views of downtown, 
even if somewhat constrained by the number of taller buildings nearby. The roof itself is nice, with a tiled floor. The furniture is high quality and comfortable, not the usual functional but uninviting metal chairs and table tops.
even if somewhat constrained by the number of taller buildings nearby. The roof itself is nice, with a tiled floor. The furniture is high quality and comfortable, not the usual functional but uninviting metal chairs and table tops.I was downtown the other day and bough a gyro from the outstanding halal cart at the corner of California and Montgomery. Looking for somewhere to eat it, I remembered the 343 Sansome was just a few blocks away. Getting a nod from the sullen security guard, rode the elevator up 15 floors. I had to roof to myself. A perfect lunch setting.
This is the sort of place I would love to discover if I was visiting another city–a hidden gem that even few locals know about. It gives you bragging rights for being an active explorer and good traveler.
As a bonus, the lobby of the building is also a public space. Again, good, comfortable furniture, and includes a modern water sculpture to settle your nerves should they need settling. Easy access to the lobby right off Sacramento St.
This is another example of free views in San Francisco, such as this panoramic view of Golden Gate Park or the view from the DeYoung museum observation tower.
They are called hidden gems for a reason: you have to know where to find them.
They are called hidden gems for a reason: you have to know where to find them.