A good place to spend a day is Coronado Island. In the Island, separated from san Diego by a Bridge, you will find nice beaches, shopping and dinning areas, and the famous Hotel del Coronado. To get to the island you can go by car or by ferry. Ferries depart from Broadway Pier, in Downtown San Diego.
A good place to spend the afternoon is the Ferry Landing on Coronado, where you will find shops, art galleries and nice restaurants. The best way to move around are the bikes, which you can rent in the island, or bring from downtown on the ferry.