Gaslamp is THE place to go eat and play in San Diego. This area is full of restaurants, cafes, food stands and bars to suit every taste. Thanks to San Diego climate, there is a number of outdoor seating options, with good food, and great people watching. Favorite pick: Cafe Sevilla, serving authentic Spanish food, is located on Fourth St. just south of Market St. After dinner, walk down the front stairs to very popular salsa club located in the basement under the restaurant. The club is very low and dark, but occasionally one can see decent salsa dancing at this place.
Gaslamp is also an epicenter of San Diego nightlife - the place realy comes alive after 10pm, with number of clubs and late night bars, and nightlife going on seven days a week. Most of the area along Fourth and Fifth Streets, between Market and Broadway is heavily patrolled all night, making it a very safe night destination. Gaslamp.org has comprehensive listings for this area, both restaurants and bars.
Embarcadero/San Diego Harbor
Embarcadero forms very unique waterfront center of action. Within short span of man-made shoreline, one finds everything from "Star of India" tallship at Maritime Museum all the way to aircraft carrier USS Midway. This is also the place where major cruise ships embark their passengers, and consequently on it is best avoided when these behemoths sail into town. If you don't mind the crowds and impossible parking, it is quite an interesting place to watch the circus, and get a close look at these immense ships. If the ships are of interest to you, make sure not to miss the tour of aircraft carrier USS Midway, moored in south part of Embrcadero as floating museum. Allocate at least 3-4 hours for USS Midway tour, and two additional hours for a tour through Maritime museum,located 1/4 mile from USS Midway. To make your day complete, take a harbor cruise on Hornblower ship for an interesting perspective on San Diego skyline, close look at Pacific navy fleet and active duty aircraft carrier (if one is in the port at the moment.) There is discount for buying harbor cruise and museum tickets at the same, inquire during purchase. Place to eat on Embarcadero: Anthony's Fish Grotto, located just to the north of cruise ship terminal. Anthony's offers both sit-down restaurant and take-out window, try the "scallops and chips" for quick lunch.
Old Town San Diego
Old Town San Diego is the place to look at the roots of this area. It is a well preserved state park, with number of historical buildings either housing period-correct stores, or restaurants and art galleries. There is also 18th-century settler cemetery, with some interesting stories attached to the grave markers. Old Town San Diego is located about 4 miles from downtown area, on east side of I-5 freeway, directly opposite of San Diego airport. Allocate about 2-3 hours to see everything. Old Town is easy to visit even when you have only couple hours in San Diego, due to proximity to airport.