If you haven't been to San Diego before, you're in for a treat - it's beautiful and has the best climate in America.
The two GA airfields are Mongomery Field in the outskirts of the San Diego and Gillespie Field at El Cajon about 10 miles NE of downtown.
There are lots of schools/FBOs at both. I've rented PA-28s or helicopters in recent times so can't help with C172s, but I've used several places over the past 20 years and never had any problems. It's a fraction of the cost here but prices still vary so it's worth shopping around.
Baja
I've never asked, but I suspect SFH to Baja might be difficult for insurance and other reasons. Years ago, someone gave me free use of a Stearman while I was staying in Texas near the Mexican border. The only restriction imposed (and emphasised firmly) was not even to fly over the border in case I had an engine failure and had to force land in Mexico. If you are allowed to fly across the border, don't forget to check the Mexican licence requirements.
Other flights I'd recommend ~
Point Loma and Coronado: San Diego bay area is stunningly beautiful. I haven't done it since 9/11 so don't know if there are any restrictions.
Oceanside: A short flight up the coast. Once clear of SD, you can fly low over the ocean if that appeals.
Catalina Island: It's challenging the first time, but worth it. The threshold is on top of a cliff so you really have to concentrate on the numbers and keeping the picture when descending on short finals. Then, just as you touch down, half the runway disappears and you'll wonder if you'll stop - there's a hump which obscures your view of the rest of the runway! Having landed, you can treat yourself to a buffalo burger in the airport cafe.
LAX: If you want the experience of landing at a major airport, LAX is only about 100 miles away. I've landed there in a helicopter a few times, but I don't think there'd be any problem with fixedwings - subject again to any post 9/11 restrictions.
La Jolla is worth visiting (by road) if you have the time. It's a very pretty town, only about 20 minutes North of downtown SD. Drive up to Soledad summit (5 mins from the town) and you'll have a superb view of the San Diego bays. If you're there in the evening, watching the sun set over the Pacific is wonderful.
Hope this helps a little.
Tudor Owen
It's probably obvious by now - San Diego is one of my favourite places in the world.