The Arenales valley is one of the best climbing destinations in central argentina, if not the best. It offers everything from bouldering to aid climbing, with the majority being multipitch trad on the vertical seas of varicolored granites. A lifetime of climbing can be expected from this area, and for every trad line that is up, there are 10 more waiting to be put up. Difficulties go from 5.5 to 5.12+.
Camping is free, but you must register with the park rangers at the lower entrance, the rangers are extremely friendly and outgoing. There is sport climbing right above the ranger station and trad on the other side of the valley. There is a mountain refuge hut in south branch of the valley leading up to the landslides and cerro tupungato, you can bivvy there for free. The water running in the stream next to the hut is drinkable and is perhaps the purest and best tasting you'll ever try. However, water treatment may be necessary since there may be dead animals further upstream. (I found a dead cow in 2010 a quarter mile upstream.)
Mind the weather and the seasons, the elevation at Arenales is around 3000 meters and the valleys will get heavy snow in winter. The weather is nice enough for climbing between october and april.
Mountain Project's determination of some of the classic, most popular, highest rated routes for Mendoza (Los Arenales):