I would recommend using a boat to get to Colonia or Tigre. To get the nice touristy feel for the two of them.

An estancia can usually be booked through a your hotel and they can arrange a van to pick you up and drive you out there. Some provide a guide alongside the driver who will take about the history of estancia's and gaucho culture during your ride.

You can check with your hotel which ones they would recommend.

I went on a day trip to this one: http://www.estanciaelombu.com/ombu.htm

They greeted me with empanada's and beer.

There were tons of other people there eating outside for the tour.

Gave tons of meat, a bit of salad, and lots of wine.

The Guacho's played music, then we went to the grounds to watch them put on a horse riding clinic and do their tricks.

Good experience and fine enough food/service.

I think it is a great day trip and really breaks from the hectic streets and diesel fumes of the City.