I know it is not CA or SA but since a few people have mentioned it and I am here right now, the pacific coast of Mexico has some great destinations. I’m in Puerto Vallarta right now, great beach city destination, it is low season now so it is easy to find inexpensive housing (I’m in Gringo Gulch at the moment) and since I eat a lot of street food and buy from the local central market (PV has a great produce and meat market in the Centro downtown area near where I am staying) feeding myself is pretty cheap. Some great little cafes near Los Muertos Playa (one of the better beaches in PV).
Since the city is a tourist town, there is a lot to do, depending on how much money you want to spend, also English is spoke widely which is a bonus for me since my Spanish is very basic. I’m surprised the DN isn’t larger here, internet seems decent (where I am staying anyway) and if you get tired of a large city, many smaller beach towns are nearby, such as Sayulita (as mentioned in previous posts) to the north and Yelapa (only accessible by water taxi) to the south. My only other experience with Mexico has been the Mayan Riviera and although it is beautiful, I did not enjoy my time there, since it was expensive (in comparison to the Pacific coastal region anyway), over run by drunken foreign tourists and very aggressive local hawkers.