Belo Horizonte, Brazil
Is it easy to go on a day trip - or two - from Buenos Aires to Montevideo? Is it cheap? Does it worth the effort?
From my perspective Colonia is better for 1 or 2 days visit. It will be expensive these days but if you plan to go in january it will be around 70 USD.
Spending a day in Montevideo is not worth it, in my opinion. To Colonia yes, because the city is very small but spend a night in Montevideo and return the next day. Always check ticket prices at Buquebus Colonia Express. It is more expensive if you go straight to Montevideo and cheaper if you buy the combo Buquebus to Colonia + bus to Montevideo (more time consuming too, 1 hour to 3 hours boat + 2 hour bus).
Isadora, Merry Christmas. Yes, very easy to day trip to Montevideo just go across the water. You also need to spend more days to see the beautiful city Buenos Aires and more nice places! Check out http://www.viajeauruguay.com/ for more information.
A day in Montevideo really isn't enough to make the trek, but more importantly, the expense isn't worth it. I did the bus and ferry and I know it was over $100 and took something like four hours.
If you do decide to go, I would try to stay at least a night. I stayed at the After Hotel Montevideo and it was perfect -- cheaper than the voyage there, in fact, if I'm remembering right.
The best is to have minimum 2 days, if it's 3 better...