The distance from Mallnitz to Flattach is about 20 km, so staying in Mallnitz you may conveniently take advantage of the Ankogel ski area in Mallnitz and Molltal glacier ski area in Flattach.
Together they offer about 80 km of ski pistes and almost 20 lifts and you may try it out buying only one Karnten & Osttirol ski pass (prices: 2-days – 70 EUR, 6-days – 175 EUR).