While in Buenos Aires, there are some side trips that are worth trying. From the most to the least popular, there are among others:

 1- COLONIA (URUGUAY):  Eventhough it's in a neighbouring country, it's usually a side trip from Buenos Aires. This small city located across the river in front of Buenos Aires, has remained an old colonial town (at least part of it), which makes it really charming. Getting there is really easy: Buquebus boats depart from Darsena Norte (between Puerto Madero and Retiro). Note that there are fast boats (1hour ride) and slower ones (3 hours). Usually Colonia is an all day long side trip, and you could even spend a night there. More than that is not necessary, unless you are headed towards another Uruguayan city such as Montevideo or Punta del Este. Do book in advance as the ferries get very booked up. Arrive at ferry min 1 hr  beforehand as this IS an international departure.

 2- TIGRE AND PARANA DELTA: A very popular destination, even among "porteños", is actually a neighbourhood on the outskirts of Buenos Aires. Eventhough there are some pre-arranged tours that may take you there, it's really easy (and safe) to go on your own. Just head for Retiro Train Station, and ask for a ticket to Tigre. It costs no more than 1 peso each way. This is a commuter's train, but it is the best line in city. Tigre is the last station, about a 50-55 min ride. Another way to go would be to get a train to B. Mitre (departing from Retiro also, but it has a lower frequency) or any bus that takes you there, and then use the "Tren de la Costa". This line runs almost paralell to the first one, but has a more touristic view. It isn't as cheap, but the good thing about it is that you can get down at any station you want (San Isidro with its Cathedral and its cobblestone streets might be a good option if you feel like exploring these Buenos Aires sorroundings), and then hop on the train again heading to Delta Station (which is roughly 300m. away from Tigre Station).

What to do there? Well, Tigre is mostly about water, but not all. Walk by the coast to get a view of the area, with it's old houses, rowing clubs and restaurants, and then catch one of the many boat services for a tour at the river's delta. Many of them offer lunch at small restaurants on the islands (these restaurants are often called "recreos"). Ask for special activities, such as rowing or water skiing.

As said, not all is water.   If not into that, the local craft's market, called "Mercado de Frutos" might be worth a visit. Tigre is more about open-air kind of fun, but if you get tired of the sun, the recently opened Museo de Arte de Tigre (the local Fine Arts Museum) offers a small but beautiful collection of some of the best Argentine artists. Plus the building itself is an old piece of art. Last but not least, you could get lucky at Trillenium Casino, not far from there.