The main site to visit from Semarang would be Borobudur. It is a colossal monument and the largest Buddhist temple in the world. It was build by the Shailendra dynasty in 800 ad. After it was taken by nature and buries under a thick layer of jungle it was rediscovered by Stamford Raffles in 1814. The drive to it is very scenic where you get a good sense of everyday life outside the cities of Java. Keep in mind it is also a very long drive of 105 km or 56 miles. This trip could take up to 3 hours one way, however it will definitely be worth the trip to visit this unique site!
Jl. Badrawati, Borobudur, Magelang, Jawa Tengah, Indonesia