other
The less active should not feel left out as there are far less strenuous treks around the base of the mountain. Ask around in Selo.
Depending on the state of volcanic activity you may be able to witness molten lava flows way down the slopes of the mountain. Take local advice.
climbing merapi
Trekking on and around Merapi is a popular activity at least when the volcano isn't spewing out hot gas and ash. From Selo, the nearest village on the north slope, it is 3 hr hike to the summit for the fit and healthy. For others it will take 4+ hr to climb, and nearly the same time to get back down again.
Carry a small pack with some food, a good flash light, with spare batteries available for purchase in Selo village, and at least 2 litres of water per person.
The track to the top is a reasonable grade and is clearly defined and easy to follow. Once through the tree line the track is less defined and the final climb to the summit after the memorial plaque is a very steep scramble over loose and broken rock. While not in itself dangerous, do take your time on the last section.
Most climbers start at about 1AM to be sure of reaching the summit by sunrise at 5.30AM. Night climbing is wise in any case, as the upper reaches of the mountain are completely treeless and the sun can fry you to a crisp. The sunrise views across East Java are quite stunning.