Mount Galunggung is the headline landscape around Tasikmalaya, an active stratovolcano west of the city with a broad crater and lake reached by a long stair climb. The 1982 eruption sent ash high enough to damage jet engines, and that recent violence gives the place a sharper edge than a pretty highland viewpoint.
Go in the morning if you can. The crater is exposed, clouds build quickly, and the return journey is easier before city traffic thickens.