All good stuff from Doc Savage. Whilst the buses are great and very comfortable to save time I would just fly....the problem with the buses are they aren't very quick and you don't see anything in the dark anyway.
Iguazu, Mendoza and Bariloche are all good suggestions and are easily accessible by air.Salta is also another good place to consider. They are all well served by package tours from BA if you don't want to organise it yourself. I would choose two places so either Mendoza for the wine tours, Bariloche/Salta for the scenery and walking or Iguazu for the falls.
You can do Colonia as a day trip from BA.