One of the best things about Sofia is that you can enjoy city activities and nature all in the same day. Only 30 minutes by car public transport spending a day inVitosha Mountain is a great decision if you want to escape from the city. There's a great walking route leading to the beautiful Boyana Waterfal strarting from Boyana neighborhood. This trip can be combined with the visit of the Boyana church - a medieval Orthodox church included in UNESCO World Heritage Sites.
There are cable cars starting from Dragalevci and Simeonovo both arrive high in the mountain. On foot to the highest peak of Vitosha - Cherni vrah or The Black peak is about one hour easy walk. On top of the mountain for ski lovers there are plenty of small ski cables following the main skipists.
If you're into water-based activities go to Pancharevo Lake and hire a pedalo. There is a nice path which circle the lake with restaurants ovelooking the water. During te summer there is a waterpark with mineral water.