3 weeks Argentina Aug-Sep

Well, Iguazu is a spot, that's fine
but Patagonia itself is vast (and very far from Iguazu). Which part of Patagonia?
do you like to see the whales? (Peninsula Valdes, Puerto Madryn)
the glacier (still further, El Calafate)?
the lake district, with lakes, mountains and woods (on west, Bariloche, San Martin de los Andes, Villa La Angostura, Villa Traful)?
the southernmost city on the world? (Ushuaia, what a place, I enjoyed so much going there last February) (but I enjoyed every place that visited in Patagonia, every time I went there)

every spot far from the other. Those are the main and more traditional places. About lake district, I prefer San Martin de los Andes. Smaller than Bariloche, but from Bariloche is easier to visit many nice places, that's true.

From August 16 to 26 you could attend the Tango Dance World Championship. Obviously, it's not worth going every day, but a couple, specially when they hold Salon Tango category.

If you arrive in or leave from B.A try going up to Tigre on the delta to the north of B.A.Take a boat trip around the inlets a fantastique place.

Depending on how much time you have and how fast you like to travel (and your budget - flying obviously costs more but allows you to fit more in), I'd recommend the Northwest - Salta and Jujuy provinces. They're absolutely gorgeous and just another distinct part of the country...

You can only withdraw pesos from atms...sux. Iguazu is incredible...you will get wet. You dont need to buy a tour, just go.
It cost like 20 pesos to get in the park. Also if you get tired of BA you might want go on a day trip to colonia, Uruguay on the buquebus (high-speed ferry).
Its a nice place to chill out.