It depends on how your getting around (car, taxi, transit) because downtown is pretty busy during the summer weekends and there is a lot fun and interesting places within walking distance once your there. Hillcrest is within 3 minutes driving distance or so from downtown as well. 8-9 hours isn't a long time to see things unless you'll be hanging out in a certain area of town or have a car to drive around.
Check out the Gaslamp district downtown if you get the chance and there are a lot of nice restraunts with outdoor seating as well.
In the Gaslamp area you you could catch a Padres vs Giants game at Petco this Saturday if your in to baseball.
If you like good burgers try Hodads downtown rated highly on the Food network
Balboa Park is always fun and pretty much neighboring Hillcrest not to mention the ...
which is in Balboa Park, and don't forget the Air and Space Museum is there as well.
Old Town is really a treat and you can get there within minutes from downtown by car or Trolley.
Little Italy is a good place to find good restraunts and sometimes street fairs next to downtown.
Seaport Village next to the Midway and Marina area and the 992 bus goes from the cruise ship terminal to and from the airport if your using the transit
Don't know when you'll be around the airport but you may be able to catch the once daily BA
777 from LHR
that arrives in SAN
at 6:15 PM
[Edited 2011-07-13 00:02:01]