When planning my travel, if there are any UNESCO World Heritage Sites I can visit, that usually influences my itinerary. Not all sites are created equal. I'm always a bit suspicious when I see an entire city or district inscribed on the list.
The historic district of Colonia del Sacramento is a charming place to spend a few hours to relax and walk around. It's not going to impress on the same level as an Angkor Wat or a Grand Canyon, but it's clearly a bit more than just a couple historic buildings.
You can find a couple of antique cars like this one if you walk around. I had the impression that there were a number of these cars spread throughout the area, but that was wrong. There's a couple, but not many. Explore the area and see what you find!