The still active cone of Mount Batur is a relatively young and small volcano located in a middle of the huge crater of its grander ancestor: diameter around 14km. Mt. Batur is located in Bangli Regency, about 1.5 - 2 hours drive from Denpasar. Also inside the rim of the original volcano is the famous Lake Batur. The Lake is beautiful in the early morning as the sun rises above the volcano's rim and the fish provide a local delicacy.
Climbing to the rim of the volcano is worth the effort, the fresh mountain air and breathtaking views in all directions makes this trip a memorable moment. The trek typically starts from Toyabungkah village, located on the foot of the mountain. This is typically a sunrise trek; it will depart very early (approx. 0400) and will take about 2.5 - 3 hours to the top and around 1.5 - 2 hours to go down. It is highly recommended for you to get a trekking guide. You can go with one of many local guides in the area and get reasonable prices by bargaining it. You can also find guides in Toyabungkah Village under the Association of the Mountain Trekking Guides. The normal and reasonable price is around IDR 150,000 - 200,000.
Of course there are other choices, some travel companies in Ubud offers trekking trips to Batur Volcano (you can ask or arrange this trip in Ubud Tourism Information) with the price varying from US$30 - 45 depends on what's include during the tour. This will be a better choice for some as the Mt. Batur areas (Kintamani) is often experienced as a day trip from Ubud.
FYI: There is also a road in the crater which is in good condition, making it easy to explore th lakeside and active cone area. There is also a route up from Penelokan village down to Kedisan - Toya Bungkah - you end up to Songan Village.