The biggest volcano in Bali is Gunung Agung (Mt. Agung), which is located in the district of Karangasem in East Bali. You can see Gunung Agung from all over the island on a clear day and scuba divers often use it as a reference point, when out in the water. Gunung Agung is also the holiest place in Bali, with Besakih, the ‘mother temple’, on the SW slope.
There are 2 established trekking routes to get to the summit. The first is from Pura Besakih, meaning a 25 hour drive from Kuta, and a long climb lasting several hours to get to the summit by dawn. This route gets you to the highest point on the crater rim. Guides are available at the base and charge anywhere from $50 for a group of 5 upwards.
From Pura Pasar Agung on the southern slope you can hike without a guide, but there are a few guys hanging out just in case. The climb is a straight shot up a 40 degree slope. No bends or switch backs.
Climbing Gunung Agung is very hard on the knees. Don’t try this in the wet season (Nov-Mar). If you do climb Gunung Agung bring 4-5 liters of water per person and food.