Colonia is the antithesis of Buenos Aires (BA): while the Argentinian capital has grown into a modern city of millions, its Rio de la Plata neighbour has turned into a quiet backwater. BA looks to have taken the better road economically, but Colonia has beaten them at least on one aspect: it has gained world heritage status, something BA still only can hope for.
I 'did' Colonia the way most people do - by ferry from BA. This is a very easy way to pick up a new country also, although I can't really say that I've learned to know the soul of Uruguay after a one-day visit. I didn't even touch Uruguayan pesos: I tried to get hold of them but all the banks were closed and the only ATM I could find didn't like my card. So I just paid in Argentinian pesos and even got the change back in that currency. One thing is different though: there's a one-hour time difference between Uruguay and Argentina (although that's only temporary)!
So what's to see? The main thing that struck me was the general laid-back atmosphere, softened even more by the leafy streets and abundance of flowers. One just has to walk around slowly, admire the nice little old buildings and have a meal or a drink. A couple of houses are turned into little museums, which are worth a look. In one of those I encountered a fossilized Glyptodon - that's a very big and scary-looking cross between a turtle and an armadillo ("about the same general size and weight as a Volkswagen Beetle", according to Wikipedia).
Colonia is generally a nice little town well-suited for a leisurely Sunday outing. Similar sites would be Macao (also a WHS) and Paraty (hoping to become one soon), although I rate these two higher than Colonia.