Sara's place is conveniently located on the side of the river, from there on you can walk straight in to town. And the area itself is very nice as well. Try the local market, they've got really nice bars there for breakfast, lunch and diner - and not very expensive.
The apartment itself was nice, quite cosy, and all you need for a weekend Madrid. Beware that if you're the one who's sleeping on the bed in the living room, that you might need to bring some earplugs. It was pretty warm so I had to open up the windows, and the city gets quite loud early morning...
All in all very good and I would definitely recommend it!