Corner rooms are huge and often on specials using VISA cards. Recommended! As to early check-in or late check-out, if available it's always possible - if not, the Lounge has people willing to store luggage, there is a shower one can use to clean up, etc.

Upgrade to Executive Floor? Don't bet on it - if available, they can do - but as a Diamond I have usually gotten a room upgrade and Exec Lounge access, not Exec Room (don't give those corner rooms short shrift!)

As for Iguassú, the Argentine side is much larger and one can spend 2 - 3 days exploring. But, I advise going on weekdays, as it really fills up with tourers (add Brasil and Argentina weekenders) weekends. I personally prefer (absolutely!) the Sheraton (it's on the edge of the park, and you get in early to see monkeys, toucans and macaws / parrots before the crowds come in from the main gate, and ditto late afternoon.

We left bags at the Bs As Hilton while we went to the Falls for 3 days - nice solution! And, the Hilton is easy walking distance of the Buquebús (take the fast ferry, and of course, your passport,) the best way to get to Colonia del Sacramento for a nice day out (again, weekdays are best, though not Mondays when lots of places in Colonia are closed.)

Puerto Madero is within easy striking distance of it all, but is more secure than most of Bs As, as the Marine (Navy) Police rule here, not the city cops - the Navy Police are not corrupt and broach no crime. And if you like beef, stop in at Cabaña Las Lilas (within a block) for some good bife - tender, grass fed and flavorful. For a traditional place, it's a longer walk to the Asadór Criollo Las Nazarenas, not to mention many other restaurants. Ay, ¡Si!