Diogo & Filipa's place was perfect! I stayed there with my sister for 5 nights. It is based in a quiet area called Saldanha. The flat is about a 5 minute walk from the metro station, which can take you more central within a matter of 15 minutes. It is also on the same line as the airport, which took about 20 minutes on the metro.
The flat is also close by to some restaurants and shops which are useful. Check out the Paleo Kitchen (5 mins away) for delicious healthy breakfasts.
The flat itself is very secure and there is a lift to the floor. It was extremely clean and tidy.