A walk along the Santa Monica pier is a nice diversion if you have some time to burn. The pier has lots of souvenir shops, junk-food vendors, arcade games, and rides (roller coaster, Ferris wheel, etc.). It is definitely a little bit rough around the edges, but fun nonetheless.
Next to the pier you will find a fairly nice section of the beach and a path for biking and roller-blading. A few blocks away, you will find plenty of nice restaurants and excellent shopping. I walked from the pier to the Third Street Promenade and spent a couple of hours browsing the shops and listening to the street performers.