Some Indonesian airports simply closed, as airplanes couldn't escape the "stoney-snow." With their engines secured to reduce the risk of damage, many planes ended up covered in the gray ash. In all, the airports of Surabaya, Bandung, Yogyakarta, Solo and Semarang were completely closed, but flights to other cities were cancelled depending on flight path.
While Indonesia is no stranger to volcanic action (there's about 130 active throughout the country) and the majority of residents around the mountain were evacuated just hours before the eruption, there was still some devastation; roofs collapsed from the heavy soot which traveled 300 miles.
Although flights have now resumed and airports are again open, the activity of Mt. Kelud had been ramping up for about 10 days prior to last week's action, and the volcano is still technically erupting. There's no telling what might happen in the next few days. We can only hope brains like this are closer to predicting this type of vacation spoiler.