I agree. Colonia is beautiful but it's a very small town, in a couple of hours you're done visiting. Montevideo is an actual city with nice areas and things to do (albeit in early January the whole Southern Cone comes to a halt), and it also has a nice beach. On the new fast ferry ("Francisco") it's a 2-hour journey from downtown Buenos Aires to downtown Montevideo. Buquebus offers Business and Economy Class. Having travelled in both, there's no big difference, so you might want to save some money and just book Economy. On a side note, Uruguay tends to be more expensive than Argentina.

I also agree with the fellow member who said in Mendoza you should hire a chauffeur. There are many tourist traps and a good chauffeur should know where to take you.

Another, easier option is to book a weekend in a estancia off Buenos Aires. They usually have hotel pick-ups and it's much closer than any of the destinations we've mentioned in this thread.