I would say two days and one night is enough to see Iguazu. I did the boat ride, two different walks and a tour on a bus through the jungle. It is a beautiful place and I highly recommend you lash out and stay at the Sheraton. The Sheraton is due for an upgrade but it is in a amazing position.

Weather should be good but be prepared to get very wet when you are around the falls.

Make sure you go to a steak restaurant in BA some of the best tasting meat in the world!