pfft skye! shame on u for not telling me ur going there! I have been going down there on and off since i was 9. My parents own a condo down there (dunno about rates and availability though ><) www.sdoceans.com and its about 100 feet off of pacific beach (i think thats the right one). I grew up (*hides*) fishing off of those piers. Didnt usually catch much--macarole (spell?) or two. But I did see dolphins and a whale and got to pet a wild pelican (fed it a fish while doing it :)
Seaworld is great, when I go down to san diego it is something I always try to get to. My favorate part of it is the manta ray tanks--feeling them suck the squid off of your hand and being able to pet their smooth, sleek body is way beyond cool. I even have a picture of me when I was little being kissed (licked) by the origional shamoo
(Oh if you go to seaworld a bit of advise..they have year long passes for not much more then one entry fee--so if you think you might go more then once I advise getting that. They also have a monthly payment plan for the year passes. You can sometimes find passes for cheaper on ebay for seaworld and disneyland as well)
The SanDiego zoo is very nice and quite famous. I would definately stop in there. It beats hoogle zoo hands down.
One thing I am surprized no one has mentioned is the SWAP MEET!!!!!!!!!!!!!!!!!!!!! omfg i *L*O*V*E* it there. oooohhh baby on a sunday all the people and the deals 

Tejuana (i think i spelt that right...oh well) is fun..but as good as it was when i was growing up--well deal wise. There are still alot of good deals and always alot of liquor down there (if your going to get liquor make sure u ask how much u can legally bring over or make sure you have a good poker face cuz if they suspect u bringing anything over your car will be searched--and that takes a long time) Another note, and i cannot stress this enough, make sure you get insurance for your car for while ur in mexico!!! There are alot of places that will offer it right off of freeway exits before you hit the boarder. The drivers down there have big grills on their cars to *ahem* help them drive around in traffic. I have seen a fair few cars just get wacked and no one even cares. and as the other person said dont drink any water while your down there--and also remember that many of the toilets dont flush toilet paper so there are lovely (stinky) baskets next to them to put the used paper. There is a nice store down there that u can get some good deals (just remember currency conversion price though). I will see if I can have my mom remember the name of it.
Oh yeah somethign else we like doing--lol but there is a good $1.00 chinese food place there. My parents love it! they go everytime and get 3 diff items and their food is only like $3 each. :)
Pier 1 clearance store is down there too and usually has some good prices--good for your house or to have a few emergency wedding presents/family gifts/etc on hand.
I am pretty sure there is a website that tells of any special events going on (not sure when the sand castle competitions are..) and if i find it later I'll let you know.
but at the website www.sdoceans.com there are pics there of my parents condo and also of some of the stuff I have mentioned here--and more info there on other stuff 
Edited by RATTISSIA