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You can't say you've explored West Java unless you've gone climbing Anak Krakatau. The climb is rather challenging, though - the ground is unstable and crumbly, having been recently laid down by Krakatau's eruptions. The trail going up the mountain is exposed to the elements, meaning that you'll be at risk of sunburn unless you follow some basic sun protection tips.
The volcano is the remnant of an 1883 eruption that shook the world; even now it's not safe to go up unless you keep up to date on early warnings for volcanoes in the region. And Krakatau is only one of your options, if trekking active volcanoes in Indonesia is your kind of thing.
Your guide was at Anak Krakatau last year, a guest of Club Bali Hawaii Resort and their Krakatau transport service. For pictures of your guide's trip up Anak Krakatau and back, check out this image gallery: A Tour of the Anak Krakatau (Krakatoa) Volcano, Indonesia.