I only spent a day in San Diego, on the seafront of Mission Beach. I'm not a beach person, but it was a beautiful and vibrant place to spend a day.
The seafront is braced with the sort of houses that I can imagine making very popular party venues for college students. The beach is covered with people enjoying the sun and the sea, and Mission Beach frequently plays host to various MTV shows filmed in San Diego.
The area around the beach offers shops and diners, and in the evening I watched a live concert at Cane's Ballroom on Ocean Front Walk.