Uruguay is a good place to spend a few days.

Montevideo is enjoyable for just wandering around a bit and having some good food. If you want an international luxury resort type setting, then Punta del Este would be a good choice(with even a Conrad Hotel available).

From Buenos Aires you can do a nice day trip by fast ferry over to Colonia del Sacramento in Uruguay. It's a very attractive town with lots of old colonial style buildings and cobbled streets(world heritage city). Leave BA early in the day and have time for wandering around town and a nice lunch and you can be back in BA in time for dinner.