This is probably the most spectacular climb in Indonesia and probrably all of Southeast Asia. The view from the summit and crater rim is mind-blowing. I climbed with a group of Indonesian students but could also be done easily independently, we went via a trail that goes around the gate. It will be hard for me to describe but its past the road the splits off to the main gate at Sembalun Lawang. Stay on the bus and go further down the main road there is a small shop across from the well worn trail that goes between two houses and has a box out front for donations to "keep the trail clean" either they are not collecting much money or not doing a very good job of cleaning. In any case the follow the most worn trail through fields it make a left and then after a bit crosses a dry stream bed and merges with the main trail there is a small sign saying Rinjani national park in Indonesian. once you are passed the gate you will have no problems no one cared I was hiking out front most of the time with my big bag clearly full of gear as I would be if hiking alone and no one said a thing. So if you have the gear you can do it just avoid the check post, I've also heard of people talking there way through the check post without a guide as well if you would like to try your luck.
Do It Yourself Information
Water is in short supply on the way up from Sembalun Lawang to the camp on the rim at Plawangan Sembalun where there is good spring water, so bring plenty with you there are some water holes but they are not particularly clean looking so I'd use a good filter. From the camp at Plawangan Sembalun on the crater rim its a steep climb on loose volcanic ground along the ridge to the peak so be careful and stay away from the edge. After the summit return to the camp and then its a deceptively long hike down to the camp at the lake side. There is fresh spring water here and even better a hot spring where you can soak your aching muscles from the early morning summit climb. There is no water on the rim at the Plawangan Senaru camp but the view from there is impressive with the lake, Rinjani peak, and volcanic cone all in a line. If you don't want to camp here it's possible to make it from the lake all the way to Senaru in one day. The main reason to camp here would be for the sunset view.
There is bus service to and from both Sembalun Lawang and Mataram, as well as Mataram and Senaru but no service between Senaru and Sembalun Lawang