The best hostel in San Luis is this delightfully renovated house named after the flowering fruit of the nopal cactus. The hostel has good dorms with bunk-beds and, for those with a love of high places, triple-decker beds! One of the dorms is female-only. There's also a kitchen for guest-use, free use of laundry facilities and a roof terrace.
The downside is the bathroom shortage – it might keep you hopping on your feet at peak bathroom hours.
The same friendly crew behind the hostel also organize decent, budget tours of the region.