Appears your best bet is getting a 2for1 rate at the HI downtown, which was a great deal when I used it back in early February. As noted in my earlier post, take the subway from the airport to Nuevos Ministeros (end of the line) and transfer to line 10 going north one stop. Take the exit at the back end of the train and after the turnstiles, head right along the tunnel. You'll emerge into a below ground level courtyard and see the entrance to a shopping mall straight ahead. Go into the mall and up the escalator to street level, then look for an exit on the north side of the complex. Once outside, you should see a cellphone store on your left, so turn left along the sidewalk and you'll see the green HI sign just up the street. Hang another left to get to the entrance.

Get yourself a subway route map at the airport station. You'll find line 10 will get you to most of the spots you want to visit with one transfer (likely at Tribunale).

There's a neighbourhood tapas bar across the courtyard from the HI where you can have an inexpensive dinner and beer if you don't so indulge down in the heart of the city.

True the HI is a bit dated, but it is well maintained and they do treat PC elites well. The lobby has just been renovated. Unless there's a football game or conference at the centre across the way, you'll find the place very quiet on the weekend.