This is a tricky one to explain, but in short: it is always more advantageous to pay in pesos... so avoid paying in credit card as much as you can, and remember to bring cash to exchange! Now for the why: There are two exchange rates for the Argentine peso: 1) the "official" rate is the one you would get if you paid with your credit card or exchanged at a bank. The rate is about 100 pesos for 1 dollar. This rate is typically not available for argentines wanting to buy dollars; 2) the "alternative" rate is the one you would get if you exchange foreign cash (dollars/euros) into pesos via alternative ways. This "alternative" rate is about 200 pesos for 1 dollar... and now you know why it is more advantageous :) NEW!!! Looks like now credit cards will give you a "preferential" echange rate, so you may be saved from using cash all over. Please check out this link and stay informed of the latest! https://buenosairesherald.com/argentina-101/foreign-tourist-dollar-everything-you-need-to-know-about-the-new-preferential-exchange-rate
There are exceptions to the "all cash" rule, where you will have to pay by credit card: a good example is flight tickets, where you will pay by credit card if you are booking directly through the airline. However, most businesses like hotels and car rentals will take cash (aka immediate 50% discount) If you do run out of pesos (or dollars/euro), most places will take credit cards - and you will pay at the "official", more expensive exchange rate explained above. Last word of advice - some taxis in Buenos Aires and most certainly in Cafayate won't take credit cards!
While Cafayate is small and walkable, there is a lot to explore around the area and roads are stunning. However, although there are no ride-sharing apps (e.g. Uber, Lyft), the town has taxis (or "remises") and most activities can be booked to include transportation. If you do decide to rent a car, you can do so at the Salta or Tucuman airport and drive up (most international companies (e.g. Avis, Alamo) have operations in Argentine airports) or pay a premium for these companies to drop-off the car in Cafayate.