La Jolla is my favorite for watching the sunset. The light bounces off the cliffs and its spectacular.
Torrey Pines has hiking trails down to the beach that make for great coastal hiking.
Coronado has a nice clean beach and you can rent bikes or surreys and bike along the coast.
Pacific Beach has beach volleyball, lots of people in their 20s, with many bars and restaurants nearby.
You can get directions to these (and other) San Diego beaches and see pictures of them at this website:
http://www.sandiego-romantics.com/romant��
La Jolla Shores
Torrey Pines