Northern Argentina is characterized by a climate with a wide temperature range between day and night. So whether you are traveling in summer or winter does make a difference at packing time.
In your luggage to travel to northern Argentina you cannot miss sunglasses, sunscreen and cocoa butter and a hat or cap, whatever the season. For excursions pack hiking footwear or boots with rubber soles, and slippers or shoes with thin soles for moments of relax.
Wear comfortable slacks as excursions often include walking parts and / or horseback riding. Also include a warm jacket because the nights are cool in summer and cold in winter.
During summer you should bring a bathing suit and sandals, as well as a waterproof jacket, because from December to March it is the rainy season.