There is enough to do to fill several days. A two night trip to Iguazu Falls is highly recommended, as it takes a full day to do a tour of the Argentinean side of the falls. I recommend taking the boat ride on the river below the falls, but be prepared to get completely soaked (waterproof bags are provided for clothes, shoes, etc. so wear a bathing suit under your clothes). We spent one night at the very new American Hotel and I would recommend it until the prices go up. It was about USD$120 when we were there, but they were still finishing some of the rooms. The airline put us up for the night when our flight was cancelled for mechanical reasons. We had stayed at the Hotel Cataratas for two nights, which was OK, but about the same price as the American and the American is a far superior hotel.
A free guided tour of La Recoleta Cemetery is available three mornings a week, and is very interesting. The half day city tour is good. A tango show is very enjoyable and a Estacion (ranch) Tour provides a glimpse into the life of the Gaucho, as well as a great lunch with unlimited food and drinks.
We stayed at the Art Suites Hotel in the Recoleta area, and were totally satisfied. We had a one bedroom apartment. A continental breakfast was delivered to our apartment when we called. The front desk staff were knowledgeable about the area restaurants and available tours and the neighborhood was very interesting and convenient with banks, restaurants, grocery stores, laundries all within a couple of blocks.