First-time San Diego visitors should definitely start out with the Old Town Trolley tour (about $30.00 per adult). This is a great way to see and learn about San Diego and find out what you're most interested in seeing. Balboa Park is also a safe bet because it has something for everyone with its beautiful gardened walks (free), loads of interesting museums (some free, others charge admission), a science center with IMAX ($11.75 per adult plus optional IMAX movie), restaurants, the Zoo ($42 per adult) and even a playground (free). 

The beach is another must and free (unless you end up paying for parking - then just be glad you got a spot!).  Any beach will do as there is lots of beautiful coastline: Coronado Island, the beach just near the Hotel Del Coronado (perfect fine sand, sun, typically low waves - best for sunning or splashing), La Jolla Cove (best for snorkeling on a clear day or walking at sunset for romantic views), Del Mar (has grassy areas above the beach great for picnics and families, water is good for all sports including surfing) or Carlsbad (great surfing and bodyboarding) and Torrey Pines (narrow, rocky beach, but unbelievable views and hikes - one path leads down to the beach).  

For shopping tourists, visit Horton Plaza and Seaport Village. The Gaslamp Quarter also has a lot of shopping, restaurants, a kids park and public transportation options. Seaport Village for walking, shopping or flying kites, Torrey Pines for beautiful ocean views and fun hiking trails (not super strenous, but not for those with mobility or health problems), Azules for romantic fine dining in La Jolla - ask for a window seat to see the sun set, or Fred's in the gaslamp for casual good food, the Science Center in Balboa Park, and the Wild Animal Park in Escondido.

Get a rental and tour San Diego Bay.  Try a jet ski, boat, paddle board, kayak, or a beach cruiser!