8 hours is definitely more than enough time. Manhattan Beach is a little closer, but the Santa Monica pier is always fun at sunset. There are great restaurants in either city, but Santa Monica has a more centralized shopping area (3rd Street Promenade) that would make for a nice walk after dinner. Even with commuting back and forth, you'll have a lot of time too kill. Would you be renting a car? I don't know about buses, but there's probably some kind of bus or shuttle to Santa Monica. A taxi would be pretty expensive, I think.