Blitar is a quiet East Java town with one unusually strong reason to stop: Indonesia's first president, Sukarno, is buried here. His grave, museum, and library make the town a place of national pilgrimage rather than just another rail stop between Malang and Kediri.
The better surprise is outside town. Penataran Temple is one of East Java's great Majapahit-era temple complexes, with carved terraces and reliefs on the lower slopes of Kelud. The volcano itself, Mount Kelud, gives Blitar a wilder edge, though conditions around the crater change with eruptions and access rules.
Blitar is not polished, and it will not fill a week. But if you are already crossing East Java, a day split between Sukarno's grave, Penataran, and the volcano country is much better than passing through without looking up.