It’s an astonishing peak, and sacred to Hindus and Sasaks who make pilgrimages to the summit and lake to leave offerings for the gods and spirits. To the Balinese, Rinjani is one of three sacred mountains, along with Bali’s Agung and Java’s Bromo. Sasaks ascend throughout the year around the full moon.

The mountain has climatic significance. Its peak attracts a steady stream of swirling rain clouds, while its ash emissions bring fertility to the island’s rice fields and tobacco crops, feeding a tapestry of paddies, fields, and cashew and mango orchards.

The mountain is also home to a trekking route that ranks as one of the most popular in Asia. Participants will experience amazing views of the surrounding landscape during the trek. The route is not too difficult for those who are new to trekking, with a climbing time of only about 2 days.
To make the best of the views, the final push to the top starts in the dark, in order to gain the crater rim as first light pushes back the gloom, revealing the crater lake and its sinister cinder cones like a lost valley of the dinosaurs.

The official website of Gunung Rinjani National Park has good maps, info and a useful section on reported scams by dodgy hiking operators. Note that trekking independently up Gunung Rinjani is not allowed.

According to lonelyplanet.com