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Rising 1717 meters above sea level, Mt. Batur is the second largest volcano on the island of Bali, but with a majestic view of the neighboring Lake Batur and Bali Sea, it is arguably the most spectacular. As a still active volcano with previous eruptions in 2000 and 1989, this volcano still releases hot steam through its porous surface and is slowly changing over time. The most famous activity to do here is the sunrise hike. Leaving base camp at 4am in the dark armed with flashlight, the hike up is about 90 minutes long to arrive at the summit sites. The hike is not too strenuous and is should be relatively easy for most hikers. From here, you will wait an hour or so to watch what will hopefully be a fiery and spectacular sunrise over the lake and ocean.
Although you could find the trail up and hike up on your own (even in the dark, the trail is pretty mellow and plain), the locals like to guard the trailhead as it is their cash crop. The locals will try to sucker you in for a steep price, some companies suggest anywhere from $50 - $100 USD for a single person. My friend and I paid 650,000 rp for the two of us, so $33 USD each. And I still felt ripped off. Note that that was for the Sunrise LONG hike. You go up the summit and around the dome of the volcano, and descend by a different route than the ascent. There are also longer and shorter hikes available for various prices.
If you are determined not to pay, it is very feasible to avoid the fee. Go to the Hiking Association booth the day before and scope out the trail up the mountain, from their booth, you should see the trail go past and up the hill. The trick is that the trail crosses a road behind the booth. The locals won't let you start from their cash crop booth, but if you just loop back to the road where the trail crosses, it is a pretty obvious straight shot up the hill. You can even hike the trail during the day if you like. Once you know the trail, it should be easy to come back at sunrise to do the hike on your own. That being said, there is value to the guide who knows the route, provides breakfast and transport, and provides the flashlights. It's up to you, just don't ripped off into paying more than 400,000 rp per person.