Great trip, best experience in Indonesia by far. Highly recommend the 4 day, 3 night tour to the Rinjani summit and crater lake. The hike is quite tough if you are not an experienced hiker (like us). The 4-day tour gives you the oppertunity to enjoy the views, hot spring and forest more (and makes it doable when you are just semi-sporty 😉). Rinjani Eco Tour provided us with a great and experienced guide (Sudir) and two excellent and friendly porters. Mr. Sapri (the tour-operator) explained everything very clearly in the beginning and arranged things we did not pack (gloves, hats, etcetera). The food cooked along the way was among the best we had. Make sure you have some time to heal the musle ache in the days following the tour!
Also, Rinjani Eco Tour tries to proserve the nature by planting a tree for every person who takes the tour with them and making sure all waste is properly disposposed. An extra reason to book with them!