Be honest about the geography
Parras sits alone in the desert, off Highway 40 between two cities, so real day trips mean real driving. It is better treated as the destination itself than as a hub for hopping around. That said, a few things are within reach if you have a car, ranked here by whether they are actually worth the mileage.
Around Parras itself (the real winner)
The best “day trips” are the short hops within Parras: out to Casa Madero at the Hacienda San Lorenzo, and to the different balnearios and springs on the town’s edges like the Estanque de la Luz. These are minutes away, not hours, and they are the whole reason you came. Spend a full day on the winery and the pools before you even think about driving to another town. Worth it, easily.
Saltillo
About two to two and a half hours each way by car. The Coahuila state capital has a genuine historic center, the well-regarded Museo del Desierto, a striking cathedral, and a real city dining scene. It is a fair day if you want a jolt of city after the quiet of the pueblo, but the roughly five hours of round-trip driving eats most of the daylight. If you do go, leave early, see the cathedral and the Museo del Desierto before the afternoon heat, eat in the center, and be back on the highway well before dark. Verdict: worth it, but fold it into your arrival or departure rather than doing a same-day round trip and backtracking.
Torreón and the Comarca Lagunera
About two hours each way by car. Torreón is a larger, workaday industrial city, useful mainly if you are flying in or out of TRC or need big-city amenities. It is not a scenic outing in itself and gives you little the pueblo does not do better. Verdict: skip it as a day trip unless you have a specific errand.
Cuatro Ciénegas
A long haul, roughly three and a half to four-plus hours each way, so this is not an honest day trip from Parras. But if you are routing through Coahuila with time, Cuatro Ciénegas and its turquoise desert pozas are genuinely special and worth a separate overnight. Verdict: do not attempt it in a day from Parras; build it into a longer state loop instead.
The verdict
Do not build your Parras stay around day trips. Use your two days on the winery, the pools, and the town. If you want a city day, fold Saltillo into your route in or out. Everything else in the region is far enough that it deserves its own stop, not a punishing same-day drive. For the driving realities, see getting there and around, and for more of the state, the Coahuila hub.