Penataran is the strongest reason to leave central Blitar for the countryside. The Hindu-Javanese complex was built over several centuries and is often described as the largest temple complex in East Java.
The site is open and atmospheric rather than monumental in the Borobudur sense: terraces, guardian figures, dated stones, and relief panels spread across the old sacred ground below Kelud. Give it time, especially if you are interested in the Majapahit period.