There are several possible climbs around Rinjani, and few trekkers reach the summit – most are satisfied with shorter, less arduous trips. All treks are dependent on how active the volcano is, so check the website before planning a trip.
The shortest trek is from Senaru to the crater rim, from where there are spectacular views across Segara Anak to Gunung Baru, and back to Senaru (two days, one night). For a longer trek (three days, two nights), a path continues from the crater rim (2hr) and descends into the crater to the lake, at 2050m. It is steep and scary at the top with metal handrails and some ropes but it gets better further down. You can bathe in the lakeside hot springs, and from the lake, you return the same way to Senaru.
The shortest route to the summit of Rinjani is to climb from Sembalun Lawang on the northeast side of the mountain, starting on the track next to the Rinjani Trek Centre. It takes seven to eight hours to the overnight campsite, Plawangan II, and you then attack the summit the next morning. It’s an extraordinarily steep haul up to the summit (3–4hr up; 3hr back down to Plawangan II). You then descend to the lake to ease tired muscles in the hot springs and return to Sembalun Lawang (three days, two nights).
The most complete exploration of the mountain involves a one-way trip; ascending from Sembalun Lawang, taking in the summit, then the lake and descending to Senaru – this has the advantage of getting the most exhausting ascent over while you are fresh (four days/three nights).