I was there in December/January/March/April
cool places in/near the Argentine mountains: Bariloche, El Bolson (the brewery has a climbing wall!), Esquel. Mendoza, though it's only peripheral.
cool places in/near the Chilean mountains: Puerto Williams (which will suck in winter), Chaiten (P. Pumalin is amazing!), Coihaique (take the Carreterra Austral between the two - AWESOME!)
places overrun with gringo tourists and not high on my nice list: Pucon, P. Natales, Punta Anus, El Calafate
Other places to see -
Valparaiso, Chile. Sit in the sun, eat amazing seafood, look at public art everywhere, and listen to the Jazz shop blasting good tunes to the public
Paraguay - It's one of the least touristed sections of S. America - the UNESCO heritage site Jesuit missions are awesome. I went to Encarnacion & C. del Este. C del Este is like Tijuana. If I'd had huevos I would have taken my camera out and shot the woman with a chicken coop on her head walking past the sign that said "No Live Animals allowed across the border into Paraguay" Encarnacion is a nice cool city - and as said, the missions are amazing.
B. Aires - chill and drink with Gringos or enjoy the art and culture of a capital city at a fraction of what it'd cost you at home. The Art Museum, city museum, and Teatro Colon are worth a visit.