I believe St Isaak square is the most beautiful square in Europe, I admire fantastic proportions of buildings, the monument and the Cathedral itself. And it is just 6 minutes walk from the apartment.
It is Petersburg of Dostoevskiy, his heroes lived at this district, and also arty and musical area since it is close to famous Mariinsky Theater and the Conservatoire.
You don't need public transport for sightseeing.
There is also big choice of cafes and restaurants nearby.