Buenos Aires is a vibrant big city. More then 3 Million people are living in the very heart of BA, called Federal district. But more then 13 million in total are living in the agglomeration. While i am sure you will very much enjoy BA, this city will likely drive you crazy after a couple of days. Intense trafic, noise, pollution... All the little pleasures of a LATAM metropolis ! One great way to survive and take some fresh air is to go and discover the Rio de la Plata.
Take the train from BA retiro station ( 40 mn ) and go down at Tigre "estacion". It's green and very cheap. Walking distance from the Tigre station you will find the harbor from which all shuttles leave to the Delta. We strongly recommend the "Interislenas" company which is usually departing from ramp 1. Roundtrip ticket will cost you 17 pesos or 4USD. Ask the driver to drop you at the restaurant "El Borazo" where you will enjoy the best meat in the area. The "bife de lomo" and "bife de chorizo" are simply a great moment ! The trip to el Borazo will last around 1H, during which you will be able to discover the magnificence of the Tigre. Be aware.
Last shuttle back ( above picture ) to Tigre estacion will leave from the restaurant at 6PM local time.
Or you could well decide to spend a night in the Delta. Several boutique hotels will welcome you. One of those we know is "Rumbo 90 lodge" where you will be able to relax and discover the charms of the river at night.