Pilar: The have a very cool zoo, with "free" animals. It´s called Temaiken and they bradn it as a "Bio Park".
Never went there, but they say it´s nice. 40 min drive.
http://images.google.com.ar/images?h...-8&sa=N&tab=wi
Tigre: If you want to spend only one day you can do the boat/amusement park thing, but if you want to do something better I would recomend spending a weekend at some cabins in the delta, whenever there is some nice weather (which is going to be pretty soon). Rio Parana de las Palmas is pretty cool imo, you have to take a boat from tigre and do like an hour trip to get there.
http://www.hosteriacruzalta.com.ar/ This one looks pretty nice also
http://www.hosterialospecanes.com/
A friend of mine has a house over there, we go all the time and I really love it, pretty close from BA but really beautiful and relaxing. You guys should check it out.
Colonia: Very nice also, went a million times and I really love it. I would spend a weekend.
Carmelo: Pretty close to Colonia and very nice also. The boat leaves from Tigre.
Rosario: Very nice city, things to do by the river. Better girls than here, there´s some nightlife, restaurants, etc. You can drive or take the train (mon to fri)
Estancias/ranches: There are plenty of them, lots of the pretty nearby, some not so much. I would spend at least one night. Horse riding, lots of asado, maybe seeing the gauchos doing their dutys, etc