Lago Chapala is the biggest lake in Mexico and as it is really warm there and you cannot see the other end of the lake, for me it felt more like the ocean (maybe as well because most lakes in Austria are really small). Lago Chapala is one of the most popular leisure time go-to places close to Guadalajara.
Next to the lake there are several small towns and places to visit, of which I will describe three below.
Chapala, being the most famous one of them, is the place people from Guadalajara go to on weekends, the atmosphere is relaxed and you can find a lot of nice restaurants.
Ajijic seemed to me like a mixture of Tlaquepaque with its nice, little shops selling handcrafts and Chapala with its beach promenade. I really liked it!
San Juan Cosala
San Juan Cosala, located about 30min from Chapala, has several Spas in which you can enjoy your day in the thermal baths, get relaxing treatments or let personal waiters deliver you drinks to your private flavored jacuzzi (among the flavors are redwine, chocolate and oats). And the best thing about it: for European standards all of this is really inexpensive!