Point Reyes National Seashore
Point Reyes National Seashore is one of the Bay Area's overlooked treasures. Located at the westernmost tip of Marin County, it is a reasonable day trip from San Francisco, but also worth visiting as a destination on its own.
From Highway 101, take Sir Francis Drake Boulevard west through Fairfax and western Marin county to Highway 1. Turn right, and take the first left (almost immediately). The next left is the entrance to the seashore.
Although some of the park's best attractions are accessible by car, the best way to get around in Point Reyes is by hiking. The park is crisscrossed with excellent and well-maintained hiking paths.
There are limited bike paths from the Bear Valley entrance, and of course bikes can be used on the park's roads, but single-track mountain biking isn't allowed on most trails.
Nearby Point Reyes Station is a small town with boutiques, restaurants, and an excellent pub. It makes a nice place to rest up after a day trip to the park.