Some expats who live in Mendoza recommended Trout and Wine for an organized tour. I had reservations about doing this in a group, but I think the tour actually worked out better than a private tour would have. There were only 6 of us, and the guide - an American ski bum who knows and loves wine - had relations with several bodegas I didn't know (and should have). We had a terrific time. See http://www.troutandwine.com/
If you're not scared by unpaved mountain roads with vertical dropoffs (but no traffic), I highly recommend renting a car for a day's drive up into the cordillera via Ruta 52 by Villavicencio (the original spring that bottles the famous water). You drive northwest from Mendoza capital on a decent paved road to the wall of the Andes. Then switchbacks for an hour up to what you think is the top of the mountain range, where you see the snow-capped peaks of the high cordillera, including Aconcagua, in the distance. The drive isn't difficult, but you need to take it slowly to avoid rocks and potholes. Ruta 52 loops around to Uspallata, on the main road between Mendoza capital and Chile, which is also gorgeous and makes for a fast return. All told, the drive is about 220 km, of which about 65 km is unpaved. You'll see flocks of wild guanacos and horses grazing the mountains and may encounter all of 1 or 2 cars on the mountaintop portion of the drive. This was the highlight of our first visit to the area.
For lodging, check www.apartmentsmendoza.com
. It's a short-term apartment rental firm run by US expats with very nice places in the city center, at prices much better than the 3 or 4 good hotels in the city.