If you've time to spare this is definitely a nice break from the noise of Buenos Aires.  It's a very pretty old town on the other side of the river in Uruguay.  The ferry takes an hour and it's a great place to wander or hire bikes and have a spot of lunch (you also get stamps in your passport!).  There are a couple of companies that offer the ferry service - and the Buquebus offers an 11:15am departure and 5:30pm return which will give you plenty of time to explore.