Located in Southern California, Mt. San Antonio is the tallest mountain in the Angeles National Forest (motto: land of many uses). It stands tall at 10,064 feet, dominating the skyline of eastern Los Angeles and western San Bernardino counties. It is an excellent mountain to hike up. If you only have a light pack, then it is a rewarding day hike, with a heavier pack it becomes a full days march to reach the summit where there are many cairns to camp in (but no water sources, so bring your own).
Camping at the top is highly recommended for those of you who live in LA, but need the wilderness to stay human. After a lung burning ascent with a heavy pack - the burn from the pollution haze gradually becoming the burn of hiking at altitude - you reach a barren wasteland of a summit that gives the mountain its nickname of Mt. Baldy. The only things higher than you when you reach the top will be the two other peaks of the Southern California mountain trifecta: Mt. San Gorgonio and Mt. San Jacinto. At night, depending on the smog conditions, you can see LA spread beneath you after the sun sets either as a pointillist collection of dots, or as a seething mass of fiery fog. When you wake up in the morning, the marine layer has set in and you can look out over the foggy expanse below you and imagine yourself to be anywhere in the world.
There are two ways up for those that enjoy hiking, and you can also take the chairlift for the Mt. Baldy Ski Resort up and then hike from there. Do not do this. It is frowned upon, expensive, and not as fun or rewarding.
Way 1, the easier way, starts at Manker Flats, about 1/4 mile below the ski area, and proceeds on a dirt road for a little ways, followed by a hard to find left turn onto the trail proper. From there it is a hike of 4 or 5 miles and 3800 vertical feet to the top. On the way, you'll pass the Mt. Baldy Ski Hut, which can be reserved from the Sierra Club, and costs $15 per person/night. When you reach the hut you are halfway there. When you reach the top, and are about to go down, you should take care not to accidentally go down the Church route, which is way 2.
Way 2 is more strenuous, and travels through more interesting terrain. It is 6 miles (one way) and 6000 vertical feet from one end to the other. Start a the Mt. Baldy Village Church parking lot and proceed up the hill from there. This route is pretty long and tiring, so get an early start.
As a matter of fact, get an early start no matter what you are doing. Both routes take a long time. Also, I recommend 3 liters of water per person as a minimum for the day hike, and more like 4-5 liters per person for an overnight stay. Hydrate or die.
The top is very cold, as you might expect for such an altitude: bring warm clothes. If you are spending the night, bring a warm sleeping bag. I recommend sleeping in the open or in a bivvy sack as I have seen lots of tents blow over in the wind on the top, because they have noting to anchor to (this is Mt. Baldy, after all). Remember: there is nothing between you and Tokyo to block the wind, so it can get pretty strong up there. Sleep in a cairn instead of exposed. Stay safe, stay warm, and have fun.