One of the more popular cities for tourists and students in Indonesia is Yogyakarta, also located in Central Java and just 3-4 hours drive from Semarang. We rented a motorbike from our bed and breakfast and rode to Parangtritis Beach. Along the way, as usual, we stopped and asked for directions - tip: always do so at a juice stand to refresh! The beach was crowded and full of activity, not as pristine as other beaches in Indonesia and certainly not the most relaxing. On the positive, the best grilled corn I've had was sold by an older Ibu vendor making it worth the trip.
The most famous site in Yogya is the Borobudur temple. We got up early to avoid the crowds, though not early enough to catch the sunrise. Borobudor is the largest and one of the greatest Buddhist temples in the world. My favorite were the stupas at the top of the temple and reliefs throughout. After the temple, we took a four-wheeler to the base of Merapi Mountain. Merapi is Indonesia's most active volcano with regular eruptions.
Finishing out the day we stopped at Prambanan, a Hindu temple in Yogyakarta. It is the largest Hindu temple in Indonesia, and one of the largest in Southeast Asia. It looks beautiful at sunset with the color of the stone changing with the light. Each evening there are two performances of the Ramayana ballet.