From downtown San Diego a regular ferry crosses over to the island of Coronado, useful if you are there without a car. Otherwise you can drive over the rather impressive Coronado bridge.
At the landing there are shops and restaurants and a beach with fantastic cityscape views. I can recommend Il Fornaio Restaurant for breakfast / brunch or dinner. Just don’t miss the last ferry back!
The infamous Hotel del Coronado, built in 1888, is known fondly as ‘the Del’ is on the opposite side of the island to the ferry landing, and where you can have a lovely brunch on Sunday or a rather less expensive meal on the deck overlooking the beach even if you can’t afford to stay there.