I went horse riding in the pre-cordillera around Mendoza and really enjoyed it, though one of the girls I was with who grew up with horses found it rather frustrating since the horses all have a tendancy to amble along and not pay any attention to you

Does mean that your hands are free to take loads of pics of the scenary lol
I did a full day horse riding tour which I would recommend since at lunch time the guide built a bbq in the middle of nowhere and bbq'ed some fantastic meat...with some nice red wine it was an experience in its own!
I booked my trip through Life House hostel but am sure you can book the trip, or one similar, in loads of places. Have fun!!!