Are you planning to stay overnight at Iguazu Falls? The Sheraton is the best place to stay purely for its location. Get up early and explore before the crowds, heat and humidity make the afternoon uncomfortably hot. There's a 'maid of the mist' minus type of boat trip that soaks you under the falls. They provide covers for your cameras - be sure and ask. It's about 200 steps down a rocky path to get there and back. Up north, consider Salta if you want to see the arid desert area. We did a similar trip in Chile last year and loved the geography, activities and people.
A good friend visited Argentina in September last year. Hands down, his favorite area was Mendoza. You can travel the Ruta de los Vinos to see the wine area (and presumably arrange some visits). Welcome Argentina.com has further information about the wineries and the wine route.
Patagonia is awesome. El Calafante is, in my opinion, a better gateway than Usuhaia. If you like an active vacation, think about visiting the Perito Moreno glacier. Friends liked Hosteria Los Notros.
In Buenos Aires, we stayed at the Sheraton Park Towers. It's a 15 to 20 minute walk to town, which was fine with us. We were upgraded to the butler floor and had great service and a comfortable room. As you've noted, it is NOT in the central area and is probably not the best choice for you. The Sheraton (adjoining and sharing the pool facilities with the Park Tower) is considerably less expensive than the Tower. The consistently top-rated hotel is the Alvear Palace hotel and is in Recoleta. Three or four of our friends have stayed there at different times and all highly recommend it.
Things to do in Buenos Aires? We went to the tango show at the Faena. How ironic .. they piped in fake cigarette smoke to create the nightclub atmosphere! Whether you choose that particular tango show or another one, they are worth seeing. Faena is in a hipper area but out of the mainstream area of Buenos Aires. We booked the tour to see the canal area just outside of Buenos Aires. It was a nice contrast to some of the areas in Buenos Aires proper. The La Boca neighborhood is visually exciting, touristy and worth an hour or two of your time.
IMHO, Argentina is a terrific place to vacation. One can have many different kinds of experiences and enjoy great food, wine and people.