What is there to do in the alameda island part of San Francisco ?

Magnet fishing.  Long throw line using

+50 pound lift magnet.  Military risk.

Stop and dial 911 if you see any hand 

grenade, naval artillery munitions or 

other explosive risk.  You'll need chain, 

grapnel, and +2 ton hand winch to pull

in motorcycles, sunk boats, etc. 

As said already, nothing. Alameda city is a quiet place and there are some good restaurants. Nothing happens. The police are strict. Don’t drive over 25mph. 

Alameda is not part of San Francisco.   It is an island on the other side of the bay.

In the 1800's Alameda was a summer get away community.  It is still has a great "small town" vibe.

To Mr. Smartypants very wonderful answer about Park street I add these places:

Ole's Waffle shop - Good  Brunch place

Tucker's Ice Cream - Amazing ice cream

There is so much victorian architecture in Alameda and is one of the most bike friendly and walkable cities in the bay area.  

Alameda is an interesting town.  There is a street made up almost entirely of restaurants of every kind, Park St.  (Including maybe my favorite Mexican place, Penca Azul.  And a block away the best German restaurant in the Bay Area, Speisekammer.)  There's a wonderful park (Washington Park) where you can get into a pickup football or baseball game.  There are miles and miles of pretty Victorian homes.  There's a WWII Aircraft Carrier you can visit (the engine room tour is amazing!)  There's a pinball museum!  There's a great historic theater-they added a cineplex but the old theater was restored as a whole. (And across the street the best gyro/falafal place in the Bay Area, Q's Halal.)  There's a wonderful amateur theater company, Alterena Playhouse, which has been there since 1939.  And several shopping areas that take you back to simpler times.

But my favorite thing to do in Alameda is to ride my bike.  It's Bicycle Disneyland.  Along Shoreline Dr. on the southeast edge of town with views of SF across the bay.  Over the bridge to Bay Farm Island and then along the shore, or into town along one of the 'lagoons'.  Next to the auto bridge is a special bridge for pedestrians and bikes!  In many places there are two sidewalks, one for peds and one for bicycles.

Where the old air base was they now have a monster flea market once a month.  I believe it's the biggest flea market in the Bay Area, possibly in the US.

Interesting thing about Alameda.  Alameda itself is an island but it used to be a peninsula.  They dredged out the channel to allow boats to go all the way around it.  Bay Farm Island, to the South, is also part of the town of Alameda.  And it used to be an island, but they filled in the channel to make it a peninsula.  (Most of it is filled by the Oakland Airport).  The residential part of Bay Farm Island used to be called Brooklyn.