Main ascent route to Mount Rinjani are two : from Sembalun Lawang on the eastern side, and from Senaru on the northern side. The two routes are quite different. The trail from Sembalun Lawang leads up through savannah and open mountain forest. The ascent from Senaru is a walk-up through dense mountain rain forest, which noticeably changes with the altitude.
There are various ways of climbing Rinjani. The most rewarding is a combination of the two routes. On the first day you can go up from Sembalun Lawang to Pelawangan II (2,900 m ?), a place on the north-eastern part of the crater rim which is a good base camp (8 hours).
On the second day you can start at 2 or 3 am, and ascend to the top, where you will arrive just in time to view the sunrise (3 hours). Make sure you arrive at the top before sunrise, not only to experience a beautiful sunrise but also to have the best chance of getting a clear view. Later the view is generally soon blocked by clouds coming up from the rain forest.
From Pelawangan II Sembalun to the summit the way leads up through volcanic scree. From the top you will go down again to the base camp and then you can descend to the lake, where you can spend the second night (5 hours, from the top). On the third day you can climb up to Pelawangan I Senaru (2,634 m), a place on the northern side of the crater rim, (2 hours), and descend to Senaru (6 hours). You can do it the other way round as well, but it seems better to climb the summit first and then relax at the lake and the hot springs.
Other routes: You can walk up to the crater from Torean, a small village near Bayan, on the northern side of Rinjani. The trail follows the stream that flows from Lake Segara Anak and the hot springs. You can also climb the south side from either Sesaot, Tetebatu, or from Pesugulan, a small village near Sapit. The last-mentioned route leads up to Pelawangan II Sembalun.