As part of my trip to Yogyakarta I explored the Prambanan Temple located in Eastern Yogyakarta.
It's easy to reach and is in fact just a couple of miles off the JOG airport. The entrance fee is steep at $17.40 for non-Indonesian passport holders but it makes for some spectacular sights with the Merapi volcano in the background. The temple compound is rather small and you can easily walk across in 30-45 minutes.
It's pretty difficult to resist the temptation to NOT go to an UNESCO World Heritage Site but the temple underwhelms a little in terms of value for money (given how cheap Indonesia usually is).