Been there in December-January. Was melting hot in Iguazu, while rain-snowing at Fitz Roy.
Lonely Planet has a very good “treking in the patagonian andes” book if you want to go hiking.
I did some hiking in Parque Nacional Los Alcerces (from FutalefÃº) and PN Lago Puelo (from El BolsÃ³n), both nice summer weather, PN Los Glaciares (from el ChaltÃ©n) and PN Torres del Paine (from Puerto Natales, this Chile), both cold weather, glaciars, wind, rain, some snow (in their summer).
The first more northen two are nice places, specially because they mainly get local hikers, easy day hikes, localy held camping places. The second southern two are world famous, turistic (which implies easy ways to arrange things), also not difficult hikes, except when weather says different, and they are just spectacular!
Hope it helps, if you need more practica info, just email.