The main square in town. Very pretty fountain, you can sit on the edge and get random drops on you. From time to time there is a tour bus stopping, letting a bunch of loud-and-selfie-hungry-tourists out for a few mins and leaves.
This is small, historic town is the closest destination (1-1/2 hours) from Buenos Aires to visit Uruguay. It can easily be done in a day. Once you arrive in Colonia, you can rent golf carts and drive around the charming streets and see the waterfront.