This Saturday, September 24th, is National Public Lands Day, a nationwide event that honors and keeps the promise of the famous depression-era Civilian Conservation Corps that left its mark throughout California. Today the day is celebrated by giving access to public lands for free (sometimes saving visitors up to $20), but also through numerous volunteer opportunities in communities, such as those who will be planting natives in San Pedro, and in places with majestic views, like the Alabama Hills cleanup near Mount Whitney.
If you're going just to enjoy the outdoors, a number of spots that usually require entrance fees or passes will be free. 11 national park units in California will be free in addition to units that are year-round, like the Santa Monica Mountains National Recreation Area and Channel Islands National Park. Some, however, are not, like Devils Postpile National Monument.
The busy national forests in Southern California that require the Adventure Pass will not on Saturday, but be prepared if you do enter an area run by a vendor, as they will still charge fees (for example, the Lower Santa Ynez Recreation Area in the Los Padres NF and Trail of the 100 Giants within Sequoia NF's Giant Sequoia National Monument).
Last year, 170,000 volunteers came out to help public lands and many more visited.