The itinerary you propose is simply silly. I'm not sure that it is physically possible, but it certainly isn't advisable. If you have any interest in these remarkable places, do devote some time to them.

We recently took the train from Yogya to Malang. If the train is running "normally" it takes 7 hours. Our train was running 8+ hours late. We checked on hiring a car and driver for the trip, but it would have taken longer (though we would have had more control over it leaving on time). Then, once you arrive in Yogya, it is over an hour to Borobudur.

There are 7 of you. You will need a large van or mini-bus. Does the car rental agency you found provide such large vehicles?

If you try to do something like this, I would arrange to leave your luggage at your hotel in Yogya. Borobudur opens at 6 am, and you can buy tickets at that time. Sunrise is about 5:30, so it will be light - not sunrise - when you enter. It is possible to enter before it opens but it takes special arrangements and the cost is considerably higher.

You can certainly arrange transport through any travel agency in Yogya. That would be the best plan.

Now, think about the people in your group. Will any of them be up to touring Borobudur at 6 am after a 7 to 12 hour journey by train or bus? Borobudur requires a lot of walking and climbing. A relatively quick tour of Borobudur will take about 2 hours. The drive back to Yogya will take at least two hours (traffic is just terrible). You could then have lunch, and a rest at your hotel before going to Prambanan. The drive to Prambanan is about 30-45 minutes, depending on traffic. I'd recommend leaving Yogya about 3:30 or so, visit Prambanan, then go to dinner. The Ramayana ballet starts at 7. It is 4 or 5 nights a week this time of the year out at Prambanan (which is the best place to see it). There is a dinner theatre in Yogya, but I don't recommend it.

We were in Java in November. You might find my trip report helpful. Just click on my name and you'll see a list of my trip reports. Photos are at www.marlandc.com

Kathie's given you some good advice. I'll just elaborate further.

Driving time from Yogya to Malang is about 10 to 12 hours. The night buses travel at maniacal speeds so maybe 9 to 10 hours. They are sometimes quite noisy -- movies, music -- so don't count on getting much sleep.

I'd suggest that you check into a hotel in Yogya or near Borobudur upon arrival. If you choose Yogya you could spend the day seeing Yogya and Prambanan. If you choose the Manohara at Borobudur, the hotel will take care of entrance to the monument. Then the next day you could do whatever you didn't do on the first.

Hi,
Thanks for your advice, Kathie and Marmot
Kathie, I read your report - very helpful; however there're some differences for my group (more people, less money LOL), that's why I ask more questions, and in fact was hoping that you'd reply. Thank you.
I know an exchange student who is currently in Malang and she suggested the itinerary as she had done similar trip before, but of course her trip was all organized by her school. I have similar concerns as you said - physical, timing, etc. and asked her but 2 weeks have gone and no reply. Same with the so-called official Borobudur park site, my questions went into emptiness. I will contact travel agents in Yogya re. transportation and hope that they won't be the same.
I understand there're other restaurants nearby Prambanan while waiting for the show, is it correct?
How about money exchange, besides the airport, where can I do it - at a bank, or exchange counters in shopping centers? What's your recommendation, more cash or more credit card payment?