Montevideo is the lively capital of Uruguay and a popular destination of travelers. It´s small enough to walk around, but big enough for some great architecture and bustling night life! Montevideo has a colonial past which you can find in the Old Center (Ciudad Vieja).
In addition Montevideo is wonderful situated along the coast of Río Plata and has several nice beaches. You can reach Montevideo from Buenos Aires by ferry within 2.5 hours.