Rosso's best day trip is also its most obvious one: cross the river into Senegal and spend time in Saint-Louis, about an hour south by road. Saint-Louis is one of West Africa's most atmospheric cities — a UNESCO-listed island town of faded colonial architecture, narrow streets, and pelican-filled beaches. It is a genuine destination in its own right and far more rewarding than Rosso for anyone with time to spare. The border crossing takes some patience, but Saint-Louis is well worth the effort.
Within Mauritania, the delta of the Senegal River stretches west toward the Atlantic and is an important wetland and bird habitat, though reaching it from Rosso requires your own transport. Nouakchott is around four hours north — close enough for a there-and-back if you have a vehicle, though most travellers treat the two towns as separate stops rather than a day-trip pair.