Buenos Aires trip
I just came back from BA and this trip re-confirmed that Buenos Aires is one of my favorite cities because the city is the main attraction. No real "must see art" no Louvre or British Museum that you simply have to hit no Vatican or Parthenon. Unless you consider the Recoleta Cemetary a must see.
Wander the streets. Take the subway it costs 25 cents. Enjoy the vibrancy of the city. Walk through Calle Forida the shopping street. Look at all the great buildings and recall that at one point Argentina was richer than Spain.
With that much time in BA you might want to take the fast boat Buquebus over to Colonia in Uruguay its a neat old city, and you get another passport stamp.
For my money the best dinner in the city is the steak house Mirasol in La Recova. Under the overpass from Avienda Julio 9. Great service fab wine list and walkable from most hotels.
Have a great trip.