Bali is home to two semi-active volcanoes: Gunung (Mount) Batur and Gunung Agung. These can be trekked either as part of a tour or (with the aid of a compass or GPS receiver) on your own. Batur is the smaller and easier of the two, at 5,633 feet. However, it is also the more volcanically active, with a crater lake, plenty of fumaroles, hot rocks and bubbling springs of boiling, green sulfurous water. Agung is much higher, at 10,303 feet. There are some signs of volcanism on the mountain, but not as much as at Batur. The main attractions of going up mighty Agung is the challenge and the spectacular view of both Bali and the straits between Bali and the neighboring island of Lombok. Guided tours of Batur are usually two days, including an overnight camping trip at the top so you can see the sunrise. Agung's tours are two or three days, depending on the fitness level of the group. A hardy group of independent trekkers could get up Agung or Batur and down again in a single day if they really wanted to, but staying the night and camping out to see the sunrise from the peak is strongly recommended.
Most of the beaches on Bali's south coast have become virtual colonies for surfers. Two beaches in particular stand out. First is Kuta, which is also home to Bali's main backpacker district. This beach has regular, moderate waves, making it a great place to learn how to surf. Experienced surfers usually head for Nusa Dua, with its regular waves in the 12 foot range. Few stay there, however, since Nusa Dua is also an enclave of Bali's poshest resorts and most exclusive golf courses.
The waters around Bali are rich with scuba diving sites. Arguably the best place for a scuba diver to base himself is at Tulamben Bay, on Bali's east coast. Right off the bay's pebble beach are reefs, walls, underwater rocks and a World War Two shipwreck. You can also sign up for two-dive day trips to other sites all around the island.
If your interest is snorkeling rather than scuba diving (or perhaps you like both), you should make a stop at Lovina, on Bali's north coast. This is a quiet area with a black sand beach, and a shallow reef just offshore that is rich with sea life. Pods of dolphins also visit the area almost every morning to hunt for fish. There are no organized dolphin swimming tours, but local fishermen have a side trade in ferrying tourists out to see the dolphins. You could take snorkeling gear with you and improvise your own dolphin swimming trip.