El Altar can be reached on a highly rewarding two- to three-day round-trip hike, starting from the tiny village of Candelaria, a fifteen-kilometre drive down a dirt track southeast from the village of Penipe, which sits 22km northeast of Riobamba on the road to Baños. There are three to four weekday buses to Candelaria (two on Sat, none Sun; 1hr 30min) from the Terminal Oriental in Riobamba; alternatively, you can get to Penipe on any of the frequent Riobamba–Baños buses, and once there hire a truck from the square (about $10, ask around).
A popular option is to spend the first night at the Hacienda Releche (t03/2949761; $21–25), a fifteen-minute walk from Candelaria, where you can get good meals or use the kitchen yourself (extra charge). From here, you can hire horses and a guide for the five- to six-hour haul up to the Collanes plain ($8 each way), at the foot of the volcano, where the hacienda keeps a simple refuge with cooking facilities (same prices) and running water (you’ll need a sleeping bag). You’ll pass the park ranger station en route and will have to pay the $10 entrance fee. From the plain it’s only a couple of hours up to the gap in the crater rim, from where you can scramble down to the edge of the lake in about thirty minutes. Heading back, count on taking about an hour to get back up to the plain and another three to four hours back to Candelaria; some stretches of the path get extremely muddy after rainfall, so consider taking gaiters or rubber boots.
An alternative to doing it independently is to join a guided hike here, offered by Julio Verne Travel and Marco Cruz Expeditions in Riobamba.