I would cut 2 days out of Mendoza and 2 out of Salta and squeeze in Iguazú if possible. Mendoza is gorgeous, Salta is fantastic, but Iguazú is just spectacular. There are flights to Iguazú from Buenos Aires (AEP and EPA), SLA and MDZ, in AR and in low-cost airlines -- I (still) haven't tried any of our local LCCs, but I have heard many positive things about Norwegian. About Flybondi I have heard mixed opinions, and regarding JetSmart I still haven't heard anything.

In the Northwest, the highlights are located in the Quebrada de Humahuaca, in Jujuy province. It's a 3-hour drive from Salta to Tilcara. There are many organised tours but I believe renting a car is your best option, as it allows for some independence. I'd make my base in Tilcara and explore the area from there (Purmamarca, Salinas Grandes, Maimará, Humahuaca, etc.). Purmamarca has great hotels too but it's a bit too small for my liking. Tilcara is also a small town but has more infrastructure (restaurants, bars, and so on). The main roads are in pretty good shape, but expect some gravel roads in the secondary network. Your rental car should definitely have a spare tire. Other than the Quebrada, the area around Cafayate and Cachi is also beautiful.