I have taken a day trip to Colonia and an overnight trip to Montevideo. There's no comparison....Colonia is a pleasant, relaxing day out with short fast ferry trips back and forth and it's an interesting and small place to visit and stroll around. Montevideo is 3 hours each way on a fast ferry so it's a long way for a day. More important, there's nothing very attractive about Montevideo. BA is far more interesting than Montevideo so I'd rather stay there for the day.

The other one night trip we took on our last trip which could also be a one day trip is to drive to San Antonio de Areco. I recommend a one night stay at an estancia near San Antonio de Areco. We thoroughly enjoyed our visit.