It's easy for the unfamiliar to erroneously assume LA is one big concrete jungle. The truth is outdoor adventure is more accessible than in most cities. Within 15 minutes you could get from Santa Monica to the Temescal Canyon Loop trailhead, from Hollywood to Runyon Canyon, or from Silverlake to Griffith Park. These are, however, rather popular trails on any given day. The main Runyon loop is in fact about 80% trail and 20% catwalk. If you want to try a little something different give these tracks a gander. Some of these routes take a higher level of agility, conditioning, and balance. Know your limits:
1. Coral Canyon - Jim Morrison's Cave
Park at the top of Corral Canyon Road / Castro Peak Motorway and cut SE along a section of the backbone trail. Just before you get to a large rock spiral you'll notice a plain looking hump of rock off the trail to the left. On the side is the secret "birth canal" opening. Slither through and you are in the cave of Morrison's LSD inspired reveries. How you choose to honor the occasion is a whole other adventure. GPS: 34.080502, -118.749228
Update 6/29/16: There is now a sign saying the cave is officially closed. The sign ironically will guide more people to the spot which was otherwise hard to find. It's also far enough from the entrance for someone to reasonably claim that they approached from a different angle (and didn't see the warning) or thought it was referring to something else (as the whole area is riddled with tiny caves). I definitely did not see it until I already came out!
2. Tree of Life (AKA Wisdom Tree)
The famous Hollywood sign is on a bit of a mesa. As you trace your eyes to the left you'll be looking at Mt Lee, Cahuenga Peak, and finally Burbank Peak just before it plunges down to the pass occupied by the 101 highway. On the corner of that mesa is a lone tree of mysterious origin which is the only survivor of a grove that burned up in a 2007 fire. This solitary stone pine is now honored by hikers as a place of reflection and reverence both due to the view and the indominatable life-spirit of the surviving pinus pinea. Hoodoos have been erected around the tree and an ammo box is full of musings to which you can add. There is a steep path straight up to the tree, but I came at it from Griffith Observatory, walking over the ridge-line from Mt. Hollywood, past the backside of the Hollywood sign, and finally to the tree.
3. Nazi Camp (Murphy's Ranch)
Wedged in a forested grove near Will Rogers State Historic Park are the scattered remains of a compound with an intriguing backstory which may bring to mind The Man in the High Castle. Although it's a fairly easy hike, this is a fascinating jaunt around a unique piece of history with unsolved mysteries entwined through it. Many hikers seem to stop short of the whole scene so take your time to poke around. Don't miss the abandoned barn (pictured) in the back.
Update 6/29/16: I've heard the authorities have recently been patrolling the area more. If you slip inside anything with a gate on it you are assuming a bit of risk. There are also plans to tear some stuff down, so time may be limited to see it all. Any info on this please post in comments.
4. Mt. Hollywood Summit at Night
Heaps of people climb Mt. Hollywood, looming above the Griffith Observatory, and several Meetup groups do it at night. However, nobody does it late at night. When all of the city is at drinking at a bar or Netflix & chilling you can make this easy climb up the mountain with only the coyotes to keep you company. The stunning matrix-like grid of Los Angeles stretches out through the dark revealing the scope of this mega metropolis. There is so much ambient light from the city that you can do without a flashlight even on a new moon, although i'd recommend bringing a headlamp. On anything close to a full moon a flashlight is completely unnecessary... the whole mountain practically glows. From the top you'll get 360 views to the coast, the valley, the city, and the Inland Empire. Access off Los Feliz by the bear statue.
5. Malibu Creek State Park - Rock Pool to Lake
Wedged in between steep cliff faces in Malibu Creek Park is the fairly popular "rock pool". Because of easy access this spot is rather swamped with casual hikers on any given weekend who unfortunately don't embrace the "pack it in - pack it out" ethos. Thankfully most of this crowd doesn't have the knowledge that my favorite scramble in greater Los Angeles begins right behind them.
Make your way behind the pool by either swimming (a dry bag would be useful) or by bouldering along the rock face on the far side, then continue upstream until you get to a dam. Ascend up the left side of the dam and you're at a serene little lake with floating lily pads. Along the way you'll pass through massive boulders that have fallen into the narrow canyon over the ages, which you can climb over or sometimes under. The rocks have created a few isolated swimming holes along the way. Look closely through the clear water and you'll see giant crayfish. From the lake hike up the hill to your right and connect with a return trail for your exit.
6. Parker Mesa via the Secret Loop Path
Parker Mesa (via Los Liones or Paseo Miramar) is a popular westside route, and for good reason. The sweeping views of the LA bay are worth the effort. But 95% of hikers do this as an out & back. I loathe returning the same way I came so I always sniff out the loops, and I found one here. As soon as you enter the new gateway at the Los Liones entrance you'll notice a concrete drainage to your left. Drop down and up the other side. The trail will now be evident. It cuts level alongside the western spine for a 100 meters or so and then shoots steeply up through some trees to get you atop that ridge. Now just follow the ridge all the way up until emerging directly below the park bench atop the famous Parker Mesa viewpoint. It's a strenuous climb, but you'll make it to the top in about 1/3 the time of the typical direction. Keep an eye out for the Getty Villa emerging directly below you. I like to schedule my hike up to grab sunset at the summit. The descent (along the normal route) can be easily negotiated at night as it's a fairly wide fire road. Just be cognizant of your turnoff onto the single-track near the end that takes you back to Los Liones. If you pop out on the street at Parker Mesa you'll have to backtrack about 5-10 minutes to find the narrower path going down.
7. The Backbone Trail
The big kahuna of LA hikes that few dare stretches over 60 miles of the Santa Monica Mountains from Will Rogers State Historic Park to Thornhill Broome Beach in Ventura County. I've only done the first segment, which is logistically the easiest: Park near Will Rogers, hike to the town of Topanga, and Uber/Lyft back to your car. The rest of this behemoth is more challenging due to its lack of convenient break points and dearth of water sources; however I plan to tackle it in the next couple months. Keep an eye out for the detailed blog post.
Update 6/1/16: The few missing sections of the trail have been acquired. All 67 miles await you.
Update 6/29/16: I've since done a few more chunks of the Backbone using a two car system as Uber/Lyft is not a reliable option and I didn't want to hike back. The trail actually passes right by the Morrison Cave (my first suggestion on this list), which has changed a touch since I was there last.