Get in
Two daily boat connections arrive and depart from Buenos Aires and are the main connection to Argentina. A fast expensive one and a cheap slow one. The fast Buquebus catamaran ferry costs (one hour) ca. 60 USD return and is usually quite crowded with day tourists and travel groups. Make sure you book in advance for weekends and the peak season (christmas until end February. The slower boat costs 102 pesos (from Buenos Aires) round trip, and takes about 3 hours.
An cheaper possibility is to take a boat from Tigre to Carmelo in Uruguay. From Carmelo, take a bus, they run every two to three hours from the center of the city to Colonia. Great landscape and also drops off kids from school along the way.
There are several connections by bus from Montevideo to Colonia. Be careful with people telling you that all busses are booked out. This is in general not true and a taxi is much more expensive. For the peace of your mind buy a bus ticket upfront to avoid rare disappointment.
If you want to extend your stay in Uruguay or Argentina than Colonia is a good place because upon arrival you get granted for either country a new 90 day tourist visa. So a day trip is although a good way to refresh your visa.
Get around
The old city of Colonia, with the main attraction, is quite small. It can be easily walked in a single day. There are also shops where you can rent bicycles or scooters which you can use to ride around the city or in to the country side. Streets aren't always in satisfying conditions, so keep an eye on the road, especially when it is cobbled.
The ferry terminal is about a half km south of the old city and the main bus terminal is about a km south of the city center. The ferry leaves 2 times a day (increase in peak season and on weekends) for Buenos Aires and takes 50-60 minutes for the fast ferry and 2:30 for the slow ferry.
It is possible to rent row and sail boats from the marina.
See
For a few pesos you can go up to the top of the lighthouse (faro) and see most of the city and look out over the city and the river.
Outside of the city there is a semi-abandoned amusement park with Uruguay's only bullfighting ring, it's not used anymore.
In July, during winter vacation, Argentine families drive around in rented golf carts.
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