Difficulty: medium (16km, about 6-8 hours round trip)
Access: car or bus (bus is quite long and difficult, check the "more info" link for directions).
This is easily one of the best hikes that you can get to from Santiago. It can be done as a day trip by car or by bus, however the bus ride is very long and requires getting up at the crack of dawn (the bus leaves at 7:30am, before the metro starts running). It's also possible to drive out to the little town of Baños Morales, camp, and start the trail the next morning, which we did on one of our visits there.
The hike is in a CONAF park and costs about 2000 pesos ($4 USD) per person. The beginning is a somewhat steep ascent for about 30 or 40 minutes, then you come into a beautiful valley and follow a small river up to the volcano that gives this trail its name, El Morado. Along the trail you get views of small salt flats, free-range horses and the volcano itself. At the end of the trail you finish at the base of the mountain, where you can also see the tiny San Francisco Glacier (easily mistaken for a snow field). There's also a gorgeous little lake along the way.
Access: car or bus. By bus, catch the number #72 from outside metro Las Mercedes. Tell the driver to let you off by El Manzano (about 40-50 minutes from the metro, depending on traffic.)
Another one of my favorites, El Manzano is great for wandering around and exploring the beautiful park, located on the way to Cajon del Maipo. While camping is technically prohibited, many people do start backpacking trails from this park and camp farther out alongside the trails. (But if you camp in the main part of the park you will probably be asked to leave.) The main trail connects up with several others that go way out into the mountains, but nothing is very well marked and you can easily get lost as you have to cross a small river multiple times along the way. I suggest just wandering along the trails carefully, rather than doing this as a serious hike. But it's very pretty and a nice, cool retreat in the summer.
Aguas de Ramon
Difficulty: medium (Paso Los Peumos trail - 2 hours round trip) / hard (Salto de Apoquindo trail - 7 hours round trip)
Access: car or bus (fairly easy to access by bus, but you have to walk about 30 minutes from the bus stop to the park entrance. Click on "more info" for directions in Spanish, under the link "como llegar".)
This is a great hike, but not on a hot day!! The trail takes you through the hills of a canyon, which are mostly very exposed, so there is very little shade in the summer. But the rest of the year, you can get great views of the canyon, the little river that cuts through it, and there is a small forest midway along the trail. The trail goes up and down quite a lot, with some steep ascents and is quite long. When you arrive, you should register for either the short loop trail (Paso Los Peumos) or the longer trail (Salto de Apoquindo). The longer trail takes you all the way out to a beautiful waterfall where you can have lunch and even swim in the little river on a warm day. On the way back, you can take the trail that goes over a little suspension bridge and return to the trail head via the right side of the canyon.
Difficulty: easy/medium (length and time varies)
Access: car only (click on "more info" for directions)
More info (Spanish)
I just recently visited El Roble with some friends and it was a very pretty little hike that is especially nice in the fall. The hike takes you up to the top of El Roble mountain, which gets its name from the oak trees that cover it. In the fall it's one of the few places around Santiago where you can see colorful leaves and walk through the shade. The trail can be as easy or difficult as you make it; basically, you drive in along a road that will take you almost to the top of the mountain. There are a few places to park along the way, so basically you just park somewhere and then walk the rest of the way up the road, which allows you to choose how far you want to walk. Despite the fact that you're walking on a road, it's very narrow and made of dirt, with very few other cars or visitors, so you still feel like you can enjoy the nature around you. At the top are great views of the surrounding valleys, the ocean, the nearby mountain La Campana and the Andes.
Access: car or bus (click on "more info" for directions.)
More info (Spanish)
This is another CONAF park, with many different trails of varying length and difficulty. They run throughout a beautiful valley, beside little rivers and surrounded by gorgeous views of the mountain. You can have a lot of fun exploring the different trails and there are plenty of great spots for picnics and family gatherings as well. This one is a little hard to access by the bus, as you have to hike into the park after getting off (about 2 or 3 kilometers), so it's much more convenient to go by car.