Maritime museum - visit the Star of India.  It's a cool sailing ship.  There are also several other ships to go on.  Last time there was a Russian sub you could crawl through.  A tight squeeze on some of the hatches, but very interesting.  

Zoo is great - I have season passes.  Wish you were here on the weekend.  I could get you in free.  Wild Animal Park is my favorite, but it is not close to downtown - about 40 miles away.  

Balboa Park - several very nice museums.

Sea World - you'll have a great time, and Monday's should not be crowded unless it's during spring break or the summer.

Lego Land - although it's designed for kids under 13, mini-land is really neat to see.  They were working on building the Las Vegas Strip on our last visit.  It's located in Carlsbad, about 35 miles north of downtown San Diego.

Have a great time!  Let me know if you have any other questions - I live in San Diego. 

LVS - Where Learning is Fun!  
http://www.lvsonline.com/index.html