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Cacheuta, about an hour outside of Mendoza, has a very large network of "natural" hot-tubs called the "Parque del Agua". During high season and weekends, you will pay AR$70. For easy transportation, inquire regarding Cacheuta at the Bus Terminal, Espreso Uspallata bus counter at the immediate south side of the terminal near platform 50; buses depart Mendoza in the morning (before 12:00) and return in the evening (last bus departs Cacheuta at 18:50). You will pay AR$14 for each way, and it is recommended that you purchase both directions before departing. If you're in for the full day adventure, consider bringing some food to cook on their plentiful and free grills.
If you want the natural hot springs, just go down to the river and find some rocks forming a pool. For free.
An upscale alternative is to go for the day to the Termas Hotel, about a km before", with swanky pools, masseuses, jacuzzis and an incredible buffet lunch, all for AR$80, but well worth the value. If you go, you had better book at the hotel a few days before for the package (do not bother staying at the hotel, the overpriced rooms are very cramped.