I really enjoyed my time in San Diego. It's a lovely city. I know I only had a small amount of time and probably didn't even scratch the surface. Balboa Park looked well worth a visit as we briefly skimmed it on our way to the zoo. We ate in numerous places, but Rock Bottom proved the best value and the nicest food.
The Harbour Tours were interesting, definitely take the 2 hour tour, then you see both the North Bay and South Bay tours, if you have only time for one do the South Bay one, this takes you passed all the American navy ships, when we were there two aircraft carriers were docked including USS Kitty Hawk .
A short walk from the Harbour Tours is Seaport Village which houses a nice little book/coffee shop where those who don't like shopping can go while those who do can waste a few hours. The Harbour House restaurant in Seaport Village was very good, I ate the New Orleans Catfish which was very tasty. They also stock a great collection of local brews.
The tram to Mexico takes about 20 minutes, but the queue to get out takes about 2 hours, bring plenty of water and your passport with visa attached to get back in. Tijuana has an air of danger which made me uncomfortable, but the locals were generally pleasent, bottles of Sol were about 70p and free tortilla chips, so for that alone I would recommend visiting it for the day. There are lots of bargains to be had, ceramic pottery and woven rugs seemed very good value, and with the minimum wage being $4 a day, the prices are great and haggling encouraged and fun.
The Horton Plaza which is owned by the same crowd who run Castlecourt has all the shops you would expect Nordstrum seems like a good shop, I had never heard of it prior to going to the states, but it managed to produce most of my pressies.