We spent the morning at Borobudur, the world's largest standalone Buddhist temple - originally built in the 8th-9th century although restored due to earthquakes/volcanic eruptions and some other security incidents. It was too hazy to come out in pictures, but the Merapi volcano was smoking in the background while we were there.
It was amazing, I would have loved to explore for a lot longer. It's huge, and covered with artwork depicting Buddhist stories, apparently with the world's largest intact collection of Buddhist reliefs/stories. Adding a bit of chaos, it's the largest tourist destination in Indonesia and was a school holiday of some sort the day we were there - it was overflowing with students, and it seemed like all of them wanted pictures with us and/or to interview us in English for school (this involved taking videos, I wasn't a big fan).