one of my favorite cities,and THE place to stay is the banana bungalows banana bungalows - although it has no bungalows but bunk beds.
you meet lots of different people there and you can have MUCH fun just staying at the hostel. even if some punks wake you up in the middle of the night, shouting "ROOM 6 ROCKS!!!" :-)
well... you either like it... or don't

san diego has lots to offer, like this old wooden rollercoaster. Ride it for the scare of your life - not because of it's height, but because of the creepy creaking of old (rotten?) wood.
downtown SD is a bit boring - it almost looks sterile in contrast to the beaches. well, it's okay to stroll around there: take a ride with the trolleys, see the old spanish main station and go for a walk in the shopping plaza, but pretty much that's it.

san diego is one of the surf capitals of the world, and that's why we went there. first you have to go and look for the right surf-spot: the best ones usually are quite crowded, but if you drive a bit you'll be rewarded.

just walk down the beaches of mission bay up to la jolla (spanish word, so pronounce it with a guttural "c-hhhh" ;-) )
as for localism (i.e. local hooligans who hate surf-newbies) we haven't made any bad experiences. in general people in SD are a bit more relaxed and laid back than in LA or SF.