Sofia's apartment is right in the city centre, between the beautiful Avenidas Novas and the vibrant Almirante Reis. It's about 20 minutes walking distance to Baixa and you'll find everything you need right at your doorstep (there's a supermarket right across the road and Ceia Minhota, a really good traditional Portuguese restaurant).
If you want to live like a local for a few days, but still be close enough to everything, then this is the place.