Very long painful climb above the west side of the city, spectacular views of several snow-capped volcanos. When you fairly close to the antenas at Cruz Loma, you can follow the main road to them, and from there west along the ridge to the peak, or (recommended), veer to the left along a stream bed that a track has been established along to install the electrical towers you can see there now. From there, you can pick your way along the bottom of the valley, heading generally north west until you get to a small clearing, where the double-track turns into single track. From there, keep riding until it gets too hard to ride, and hump your bike the last few hundred meters to the ridge. There is a good, hard single-track that runs along the top of the ridge (originating at the antenas, but the first section from the antennas can be very steep, and up and down, with some unnecessary climbing). Follow that single track west and up to the edge of the rocky bit marking the volcano's peak. From there you can skirt the peak as far as you like. The farthest I've been is half way around, to come down the next ridge to the north, meeting up with the Pichincha Antena's trail. A very challenging ride, probably 4 hours minimum.
Go to the Mercado San Roque just south of the tunnels on Avenida Occidental, and behind the market is a road that zigzags many times up the steep slope to end up behind the monumento de los libertadores. You can start riding from here if you like, but I usually take my vehicle (or a cab) to the end of the cobbleston section, which is a further 3 or 4 zig zags, probably 3 miles, up the hill. Whenever you hit a fork in the road, bear to your left, until your at the end of the cobblestones.