Tumpak Sewu is the reason most travellers make the long detour to Lumajang. The waterfall drops in a wide semicircle into a green canyon, with Mount Semeru often rising behind it when the weather is clear. The upper viewpoint is easy enough, but the real experience is the descent into the gorge.
That descent is wet, steep, and sometimes rough, with ladders, river crossings, and slippery rock. Go early, use a local guide if conditions are uncertain, and do not treat it like a paved attraction. The reward is one of Java's great waterfall amphitheatres, loud enough to drown out every bus ride it took to get here.