When you visit Mendoza, be sure to make a trip to Potrerillos. It's the ideal place to go in the Mendoza area if you want to spend a weekend out in the countryside. There are all kinds of water-based activities available on the lake here: I paddled around it in a kayak, an experience I thoroughly recommend!
There are also many walks you can do around the lake, giving you a chance to stretch your legs and enjoy wonderful views over the pre-Andes. The best thing to do is set off early in the morning and have a picnic at the site. You have to pay small charges to to access the paths and trails, but this is compensated for by the fact that the site is very clean and well-maintained.
Before heading off back to the city from this bucolic area, you simply must stop for a maté tea by the lake; and why not round off the day by lighting a campfire and having an "asado" together (an Argentinian barbecue), which is what many groups of people come here do.