Krakatau might have come closer than any other volcano in recent history to destroying the planet when it erupted in 1883. Tens of thousands were killed either by the resulting tidal wave or by pyroclastic flows that crossed 40km of ocean to incinerate Sumatra coastal villages. Afterwards all that was left was a smoldering caldera where a cluster of uninhabited island had once been, perhaps peace had come, thought local village. But Krakatau. Like all scrappy villains, re-awoke in 1927 and resulting eruptions built a new volcanic cone since christened Anak Krakatau.
Tours to the island start on Banten or from Kalianda on the Lampung coast. Organized tours typically cost US$100 per person. Hotel Beringin in Kalianda might also be able to organize a tour.
You can also join up with weekenders chartering boats from Canti, a fishing villages outside of Kalianda, or from Pulau Sebesi. To go to Kalianda you can take buses from Bandar Lampung buses station in Rajabasa. Or from Bakauheni ferry terminal to Kalianda. There also cargo boats from Canti, to nearby Sebuku and Sebesi. Canti itself can be reach by public bus.