Leaving 12/19 you won't get there til 12/20 at ~9:45am.

That leaves four days in Buenos Aires before flying out the night of 12/25.

Iguazu Falls can be done with only two full days but you should see the Brazilian side as well and that would, YMMV, require a Brazilian Visa.

Cost of: 15,000 Avios for Roundtrip AEP-IGR or $240 for your dates.

Mendoza is also a great city to stop for a drink at some of the famed Bodegas, winery, of the region. They also have great countryside day trips (Aconcagua highest mountain in Americas), horse back riding and more.

Cost of: 9,000 Avios Roundtrip AEP-MDZ OR $315 for your dates.

Uruguay is a great choice as well. With Pluna, Uruguayan airline, going out of business you are pretty much limited to Aerolinas Argentinas, Sol Lineas or Buquebus (boat, bus and planes).

Two examples of Costs: Colonia for ~$55 in a 1 hour 15 minute Buquebus boat ride
Punte Del Este for ~$300 for 50 minute Aerolineas Arg flight.


And that is just the tip of the iceberg for Argentine travel.

I am not sure how you like to travel and if you can do quick day trips or not.

I hope some of the information is able to help you make a decision but I don't think you can go wrong no matter what you choose.