Well we had a lovely two days in Colonia with our kids. Having read some of the negative feedback on this thread (no surprise there!) we went expecting very little. What we found was a lovely little historically interesting town which had some great little restaurants and plenty of places for the kids to explore. They spent most of one day slowly walking around the edge of the town exploring the rock pools and beaches. It was like going to a Cornish village in winter. We went to the aquarium which was very small but well presented and taught us a lot about the local species. I think that nowadays we expect everywhere to provide on tap entertainment for our kids but all they want to do is explore themselves. This is something we miss a lot in BA. Footpaths and clean rivers to paddle about in.
We stayed at the Raddison after getting a good deal on Despegar. Very good with inside pool and jacuzzi which kept the kids entertained as well. Also had a casino which we didn't visit!
Buquebus was one of these sealed in boats which didn't give the chance to explore on deck. More like being in an aeroplane which was disappointing. Only took an hour though.
The one recommendation I do have is the Calle de Suspiros which has a great little wine shop and picada restaurant.
http://www.buensuspiro.com You can feel the colonial history in the walls seeping through. Don't forget it's a world heritage site founded in 1680. Not too many places you can appreciate this here.
On the whole go there and relax and enjoy some history away from the noise and pollution of BA.