7 days in Buenos Aires seems good, but it does leave you time to do a side trip.

Early January can be (will be) very hot. If you want to escape the heatwave, a weekend getaway to Punta del Este, in Uruguay, could be nice. AR and BQB offer nonstop flights to PDP, although you might want to check fares to MVD (also AR and BQB), as it's usually cheaper to get to Montevideo and then take a bus or rent a car. I'd personally rent the car, MVD-PDP is a 1.5-hour drive through a nice highway and while there's almost no public transportation in PDP, the city is quite spread out. Otherwise the same hydrofoils that go to Colonia (Buquebus) offer connecting buses to PDP.

Iguazú will be even hotter and wet, plus you have the Brazilian visa issue if you're travelling on a US passport. Ushuaia would be nice and you'd get very long days, the sun sets almost at midnight in the summer. The same applies for El Calafate.

Mendoza is also a good possibility. The city itself is OK, the surroundings are very nice. You might want to consider staying at The Vines of Mendoza, which is a 5-star hotel located 2 hours outside the city in the middle of the Andes. I haven't been there personally, but friends who stayed really liked it.