The weather! In my opinion, one of the best reasons to go to San Diego is that it is sunny and mild almost every day of the year. It may rain occasionally, but even then you probably won't get very wet. I have never seen it pour in Southern California as it does in the Midwest.
The food is also great! If you want real Mexican food without heading to Mexico, San Diego is one place you can get it. And, if you want to go to Mexico, you can ride the trolley all the way to the border.
Plus, the scenery is wonderful! There is the coastline, with its beaches and oceanfront. Plus, the mountains are not far away. In addition, there are historical buildings that have been restored, such as the Missions.
All in all, these reasons make San Diego a great place to live or visit!
Quick Tips:
The cost of living in the San Diego area is rather high compared to much of the rest of the US, so that can cause problems for vacation budgets. However, many hotels in the San Diego area offer mini-fridges and microwaves in their rooms, making it possible to eat some meals in and keep costs down.
I also recommend looking for the best deals when finding hotels. Hotel Occidental was one surprisingly inexpensive find in the downtown hotel world of + rooms. Location also makes a difference: how much to do you care about being right downtown versus being a little farther out?
Also, ask locals for help in planning your itinerary. While some places, like the San Diego Zoo, would be on most tourist lists of places to go, other places will benefit from a local's perspective. For example, I asked the hotel clerk whether Old Town San Diego really was a good place to visit and found out that while it's known as a good place to eat, the cultural end of it doesn't really hold up. Best Way To Get Around:
Around town—There are several public transportation options: the Light rail (known locally as "the trolley", not be to confused with Old Town Trolley, a tourist service), bus, taxi, and commuter rail. What works best for you depends on where you are staying and where you are going. Fortunately, the transit authority has a website that will help you figure out the best routes to where you are going. Just go to http://www.sdcommute.com/ and click on "Trip Planning." You can also get their phone numbers (for various parts of the county) from their website so that you can call for trip planning services.
There is also the option of renting a car, but if you are staying in downtown, I would not expect that to be necessary. It might actually be somewhat of a hassle, as I am not sure of what the parking situation is like downtown.
To and from the airport—It depends on if you are traveling alone or with others. Alone, your best bet is to use Cloud Nine or one of the other shuttles to get you to your hotel, as it will cost around . Under ideal conditions, that might be the same cost as using a taxi, but the shuttle won't cost extra if they get lost going to your hotel! If you are traveling together with others, a taxi is probably your best bet from the downtown area. The bus is also an option if you are traveling light and staying along the main route of the airport bus (992 Airport/Downtown). At .25, the price can't be beat, but if you need to go off the downtown route, the wait time between buses will probably make it unwieldy.