Colonia is a day trip, best done by taking an early boat and coming back in the afternoon, and that is plenty of time, including a relaxing lunch.

Montevideo is a pleasant city.. That is an approx. 3 hour each way ferry (although I think a higher speed boat is serving the route) and makes for a pleasant overnight trip.

Mendoza - I recommend getting a driver and let your guide know what you are looking for, or you could end up on tourist hell tours. People here can guide you in the right direction for that.

Punta del Este can be very expensive and crowded in January and will be full of pretty people who need to be seen. It doesn't seem like anything that a tourist that isn't on a "beach vacation" would want to do.

I wouldn't recommend an overnight to the "deep south."