Between Yogyakarta and Magelang lies the volcanic Kedu Plain. This was clearly an important area in pre-10th century Javanese history as it contains a whole host of ruins (both Buddhist and Hindu) dating from the same era as Borobudur, and easily reached from there. If you have a car, the most accessible of these together make an interesting use of the late part of the day on the way back to Yogyakarta after you have seen Borobudur. Alternatively, if you are staying in the Borobudur area, rent a bicycle and explore these temples together with the verdant local countryside.