The highlight of Yogyakarta should be Borobudur Temple, it is well maintained, accessible to every part of it. The crowds can be huge so better to go for the sunrise tour eventhough, you have to pay more, it's worth it. Location wise quite far from Yogya town around 1.5hrs. If you want to go for the sunrise tour be ready by 330am to travel there. By the time you reach Borobudur it would be around 5am.