The sunrise hike of Mt Batur is a very popular activity you can do departing from Ubud and takes place every day, dry or rainy season. I did it in the dry season (August) and was an amazing experience, and lucky enough the day was really clear and the sunset on top (1717mt) was stunning.
Mt. Batur is one of Bali active volcano, not active since 2000 (according to Wikipedia). If you want to do this hike there are a lot of organized tours (or many people selling the same one), I did it in August 2014 (but asked again for info in 2017 and things are pretty much the same) starting from Ubud (read about Ubud here). So once in Ubud you can find packages starting from 250k rupias in many of the home stays or on the street tour operators (I won’t pay more of that, but maybe in high season is a bit more expensive, I’ve got this price in my last visit in Feb 2017). The tour include pick up (2 am usually) from your hostel, drive to the bottom of Mt. Batur, guide for the 2hr hike (give or take 30 minutes, if I remember well), torch lights, food pack (some local stuff like eggs, cracker and fruit). The hike is an easy one (can be tricky close to the top if has been raining the day before), but doing it in the pitch black night, following the dozen of lights of the people in front and behind you is an amazing experience.
Once on top, if you are lucky, the horizon will take the colors of the rainbow and the sun will make is appearance some 30 minutes after that in a stunning explosion of orange and okra shades. After the walk you’ll be soaking wet and once on top the breeze can be unbelievably chill so bring a change of clothes if you don’t want to freeze before you start to head down. From the top you can also see the lake and volcano belly and some geyser puffing out boiling steam. The walk back is nice in the morning breeze, can be slippery if it rained or in the morning dew. Once down (around 7ish) your tour might include a visit to the coffee plantation and rice terrace, which will be nice (and free in this way, if not you’ll have to pay for it).