Hi there,
Hope you have a wonderful time in San Diego, it's a beautiful city. I've lived here since 1982. Let's see, if you're flight leaves at 6:45 p.m., you need to be at airport by 5:30 at latest. You say you want to go "up the coast." To drive up the coast, you would take Interstate 5 north. From San Diego, you will hit La Jolla, Carlsbad, Oceanside and Orange County. The thing is, if you go all the way up to Orange County, that is approximately 90-120 miles (depending on how deep you go). That is a 1/12 to 2 hour drive, each way, without traffic. So, you have to be careful not to go too far. I'm not too familiar with Orange County beaches, I think you're best bet would be to just drive within SD County and see the sites. Drive up and see La Jolla. La Jolla shores is beautiful, so is Del Mar which is just north of La Jolla. You may also want to go slightly south of San Diego to Coronado. That is one of the nation's best beaches. It's beautiful. Stop and browse at the Hotel Del Coronado and hit the beach there. You won't be disappointed.