The trip Yogya-Bromo-Yogya will take too much time, Mawar. May i suggest the vulcanic area Dieng Plateau
as an alternative? Arguably much more interesting than the Bromo at that. You can do the trip from Yogya and back in one day, although I'd suggest to spend a night in Wonosobo. You can also combine the trip with a visit to borobudur.