Gunung Merapi, the dangerous volcano, near Yogyakarta

You can't climb the mountain anymore since the Indonesian authorities decided the volcano errupted way too many times and killed way too many people but you can still walk up to view some of the activity with a qualified guide. One of the hotels in Kaliurang resort, Vogels Hostel, organizes early morning trips to the volcano (ask for Mr Awuy). You can still try to climb from the village of Selo but officially climbing or trekking is off limits.