We flew to Yogyakarta for the day from Bali. It was fantastic.
We visited Borobudur, the largest Buddhist temple in SE Asia, the Hindu temples of Prambanan and the Sultan’s Palace and the old part of Yogyakarta. This is all very possible on a day trip.
It is a massive contrast to the beaches of Bali and very interesting and scenic as well. It was well worth the effort and, with flights costing only $40 each way, it was quite an economical day trip.