In 2014, I spent 10 full days in Indonesia. Of that, I spent 5 days in Java visiting Yogyakarta and Bromo, and 5 days in Bali in Ubud.

I do think you have enough time to visit Yogyakarta and Bromo, in 5 days. You can fly KUL to JOG. Spend 3 night, and visit Prambanan and Borobudur. On the 4th day you can fly in the morning from JOG to SUB. There flights are not expensive. You can arrange car and driver to pick you up from the SUB airport and drive you to Bromo. See the sunrise early next morning, hike to creator, and then after a late breakfast, have the driver take you back to the SUB airport to catch an evening flight back to KUL on the 5th day.