A favorite with every visitor to San Francisco is seeing the Sea Lions at Pier 39.  As you enter this touristy pier, head to the left.  You'll hear and smell the sea lions before you see them.  Hundreds of them have taken over a couple of docks from the local fishermen and are alot of fun to watch. They will buslly push one another off the pier, pick fights, sun bathe or just swim around.  Allow at least half an hour to watch the fun.  Best of all, it's free. 

On a cold day go sit inside at one of the cafes on the sea lion side and watch them over a coffee or glass of wine.  Afterwards, Pier 39 is worth a quick wander.  The free shows by the street performers are often amazing.  And it is a good place to pick up souveniers or unique gifts.   Harbor tours and the Natl Park Service boat to Alcatraz leave a stone's throw away.  Just turn right as you're leaving the Pier (towards Fisherman's wharf).  Only warning is that parking is expensive around here.