Sewu temple complex is less than 1KM from Prambanan Temple.
It was high noon with very harsh sunlight, so I experimented using the built in art filter in my camera to take an ‘art-line’ impression of Candi Sewu.
Visitors can walk or take a free tram ride from Prambanan to Sewu. The ride is free of charge for foreign visitors (because they already paid an expansive entrance fee to the complex).
One can finish visiting Candi Sewu within 30 minutes, just in time to ride the tram again to the exit point.
If you don’t like stone temples and relic, then you can skip all the temples around Yogyakarta, and do nothing. That is the same as visiting Seam Reap and skip the visit to Angkor park.