One I like to do every once in a while is a day trip to Tigre, north of Buenos Aires.
There's the fruit market, though it's not only fruit they sell but all kind of handicrafts, you can walk along the riverside, enjoy the view and have lunch in one of the many restaurants, and take a boat for a short ride around.
There's even an amusement park, and a casino.
If you want more privacy, you can spend the day relaxing in one of the many islands, they're like resorts with all kind of facilites.
To get to Tigre, you can catch the Mitre train at Retiro. Or, if you're more adventurous, the 60 bus.