Chui Lighthouse stands on the Brazilian side of the Arroyo Chuy, close enough to be part of the border-coast outing from Chuy. The red-and-white tower marks the mouth of the stream and the southern end of Brazil's Atlantic lighthouse chain. From the Uruguayan side, it gives the beach a useful landmark.
The lighthouse is more a destination for a coastal walk than a museum visit. Check border formalities if you plan to cross into Brazil properly, especially if you are travelling onward rather than just wandering near the beach.