I've always considered La Jolla a great place to go if you want to be at the beach. This town is located just North of San Diego and although it does attract tourists, there are plenty of places you can find on the beach where practically no one else is. When the tide is out, you can see little creatures moving about in the tide pools. One of my favorite things to do out there is to climb some of the rocks, grab a seat, and watch the waves roll in.
There is also a big park located by one of the beaches if you want to join in a game of volleyball or socialize. You will also find a bunch of little shops along the shore.