If you don't have a car, Dana Point and San Clemente are not good choices. Dana Point has a nice harbor and some really nice($$$) hotels, but really doesn't have a nice downtown area. San Clemente has a nice shopping/restaurant street, but it's a hike from the beaches. SC also has a pier, the beaches are nice, but I wouldn't want to be there without a car.
Laguna Beach is probably the best choice of the three if you don't have a car. The Main Beach area has a cute downtown with lots of shops & restaurants, and a few hotels.
It can still be pretty warm in Orange County in October and the summer crowds are gone. The ocean is chilly.
If you want to visit coastal Orange County, I'd strongly reconsider not renting a car. With a car, you can visit San Clemente, have lunch on the pier & walk around the shops on Del Mar Street. There is also an outlet shopping center, if you like that kind of thing. Drive to Dana Point harbor, get the best ice cream(Lapperts) and walk around looking at all the boats. You can also take a ferry to Catalina Island from here. You can visit Laguna Beach. Park at the Montage Hotel, walk the grounds and hang out at my favorite beach(Treasure Island). Drive up the coast to Newport, have lunch at the Beachcomber restaurant in Crystal Cove State Park and walk up and down the beautiful beach there. Visit Fashion Island shopping center, an outside mall with a great ocean view. You can also ferry to Catalina from Newport harbor. While in Newport, visit Balboa Island and walk around the perimeter, enjoying the cute houses and boats. Visit the shops and restaurants along Marine Avenue.
There are a lot of fun things to do along the coast and you'd be limiting yourself if you are car less.
Q
