If you don't want to hit the major tourist attractions, there's still plenty to see and do here in SD. I'd suggest Old Town, full of history, great mexican restaurants, museums, shops, and haunted San Diego's pride and joy, the Whaley House, which offers walk through tours. Our guests always love a day in Old Town, and it's one of the few places that still offers free parking. ;) There's also trolley access just a block off the main drag.
Another great place to spend the day is the embarcadero downtown, featuring our maritime museums (The Star of India, The Midway, The Surprise, a russian sub, etc.) and lots of public art. From the embarcadero, you can walk all the way to the Gaslamp quarter, passing along Seaport Village (another great place for unique shopping and some nice restaurants, usually some local artists on the weekends, and lots of boats). The Gaslamp comes alive at night with bustling clubs and restaurants galore (some good sushi places, spanish food, thai, japanese, persian, indian, irish, excellent fish tacos; you name it!). There are lots of boutique stores, often a couple street performers, live music, and lots to look at. There's also Horton Plaza, a 5 story open air technicolor mall that looks like it fell out of an MC Escher drawing. And of course there's Petco Park if you want to take in a Baseball game while your here, the atmospere is always fantastic, even if the game is terrible.
Our beaches are fantastic too. For your age group, I'd suggest Pacific Beach, full of young people and lots to do on and off the sand. There's an amusement park at Mission Beach too with a wooden roller coaster and Wavehouse. If you want something more relaxing, try snorkeling at La Jolla Cove (just mind the current and the seals) or exploring tidepools at La Jolla shores - they're on the other side of Scripps Pier, a nice walk along some of our most pristine beachfront. Go before 11 am for good parking and leave before 3:30 to avoid traffic. Coronado is also a great beach to hit, sparkling clean white sands and a lavish victorian hotel are its hallmarks, but there's plenty of action in the waves and a great 1950's style walk (or bike cruise- try Holland's bicycles) down orange. Bring quarters for the parking meters or go on Sunday -it's free!
Another fun place is Balboa Park. The park itself is huge and contains the San Diego Zoo and the bulk of our museums. The architecture of the museums is incredible and nearly 100 years old. It's all very ornate spanish style, with a wide walkway where performers play for handouts nearly every day during summer. There's the Japanese friendship garden and Organ Pavillion at the far end. The near end features extensive rose and cactus gardens and a Mexican style village filled with artist guilds and their wares -paintings, sculpture, glass, jewelry, etc. Parking is free, but often hard to find unless you go early, or in the late afternoon.
If your hotel has a concierge, use them. Ask them for things to do in the area, often they are so well connected that can plan a flawless day for you from breakfast to dessert (speaking of desserts, if you want to wow your girl, a trip to extraordinary desserts is a good way to start) with maps and transportation and reservations so you don't have to mess with it. Enjoy your vacation in San Diego! ^_^