Punta del Diablo (Devils' Point) is one of the best places in Rocha. There you can enjoy beautiful oceanic beaches of white sand, be delighted with the tipical town and stay in one of the cabins or apartments that people offers to rent.
Just 20 minutes by walk, to the south, there is a place called Punta de la Viuda (Widows' Point). There you can find a lighthouse and a pair of houses with red roofs.
I think there is no way to go to Punta de la Viuda by car, your only chance is to go walking through the beach. Early in the morning or late77 in the afternoon will be perfect to protect you from the sun, specially in the summer season.
When you arrive at the place you will see lots of large rocks and the lighthouse. You have to climb the rocks and go close to the fence, there is no need to pass over the fence (it's private property).
Then you will find some red arrows painted on the floor that will guide you through the rocks. Continue climbing until you go near the lighthouse.
Your cache is about 50 meters passing the ligthhouse, near the fence.
Do you see the big stone that look like a giant dice ? Well, you are very close, just 20 meters passing that big stone there is other rounded stone, close to the fence. Your cache is there.
Enjoy the trip back to the town and take a rest in one of the restaurants there.