The province of Buenos Aires is dotted with lovely old estancias (ranches) that are open to visitors - but some are irritatingly touristy. For a more down-to-earth dia del campo (day in the country), try Estancia Los Dos Hermanos.
A typical day trip to the estancia starts with a leisurely breakfast of coffee and pastries at a picnic table near the stables. When the horses are ready to go, you're off on your morning ride with the guide, exploring grassy green fields and dusty country roads. A full asado (barbecue) is waiting for you back at the ranch - there's so much steak and wine served at midday that you might need to recline in one of the hammocks afterward - followed by a longer afternoon ride.
Though it's not impossible to arrive here on your own, the unmarked rural roads can be confusing - let the ranch arranges your transportation to and from Buenos Aires.