On a sunny San Francisco day, there’s no better place to spend time outdoors than in the Marin Headlands. For more than a century following the Civil War, this vast expanse of grass-covered hills and rocky shore was off-limits to the public, appropriated by the U.S. Army as a strategic base for defending the bay against invaders. Remnants of obsolete and untested defense facilities--dozens of thick concrete bunkers and batteries recessed into the bluffs--now serve as viewing and picnic sites for the millions of tourists who visit each year.

There’s a wealth of scheduled activities offered daily within the 15-square-mile Golden Gate National Recreation Area (GGNRA), including birding clinics, bunker tours, wildflower hunts, and geology hikes. The Marin Headlands Visitor Center (take the Alexander Ave exit, the second exit after crossing the Golden Gate Bridge, off Hwy 101 and follow signs; 415/331-1540; www.nps.gov/goga/marin-headlands.htm; open 9:30am-4:30pm every day), located within the headlands, houses plenty of information, including maps and pertinent facts about all the locations listed here.

The Marine Mammal Center (415/289-SEAL; www.tmmc.org; open 10am-4pm every day; free), a popular Marin Headlands attraction, is a volunteer-run hospital for injured and abandoned mammals of the sea. Signs list each animal’s adopted name, species, stranding site, and injury--the latter of which is usually human caused. The center is located at the east end of Fort Cronkhite near Rodeo Lagoon.

Closed to the public for several years due to storm damage, the precariously perched 1877 Point Bonita Lighthouse (415/331-1540; open 12:30-3:30pm Sat-Mon) is once again thrilling those tourists brave enough to traverse the long, dark tunnel and seven small footbridges leading to the beacon. The reward for such courage is, among other things, a rare and sensational view of the entrance to the bay. The story goes that one 19th-century lighthouse keeper rigged ropes around his children to prevent them from slipping into the raging sea below.

Also within the Marin Headlands is Hawk Hill (located above Battery 129, where Conzelman Rd becomes one-way), one of the most remarkable avian sites in the western United States and the biggest hawk lookout in western North America. Record count in 1992 was more than 20,000 birds, including 21 species of hawk. The best time to visit is during September and October, when thousands of birds of prey soar over the hill each day.