Big Bus - San Francisco
Cost: 24 hours $45 USD / 48 hours $54 USD / 72 hours $63 USD
Highlights: Shows you all the highlights of San Francisco and gets you down to Fisherman's Wharf with relative ease
Must do: Sit on the top while driving over the Golden Gate Bridge
These sightseeing buses are nearly always the best way to get your bearings in a new city if you're only there for a short time, and also a good way of getting some information at the same time. I found the old school Big Bus great value and found it went to all the cities best sites. It is quite a hike from Union Square, down to Fisherman's wharf and these are the two main tourist areas in the city.
The Big Bus also helped me find things in the city I didn't know where there, like the cities alternative district and took me through the unsafe tenderloin area in comforta and safety. Going over the Golden Gate Bridge on the top of the bus in the fog was absolutely priceless.