I don't understand how Mar del Plata is the closest beach when if you are going to stay in Argentina the much better beaches are Pinamar and Carilo. They are nice little towns worth a visit for a weekend maybe, instead of just a city by the beach they are towns constructed amongst forest and dunes.
HOWEVER
Going to beach in Argentina if you have any option of going to Brazil is ridiculous. My MIL has a place on the coast and we never really go, the water is freezing, I think it's colder than swimming in Maine! If we want a beach holiday and we have money we go anywhere but the Argentine coast. The argentine coast is expensive as well, more and more Argies are going to Brazil because the rentals there are cheaper.
But yes, if you want to be able to get on a bus and go for a weekend, go to Carilo, Pinamar, or Villa Gesell. Don't go as far as Mar del Plata, it's not worth it. If you have the money instead of a bus from Retiro which is pretty awful (though if you've ever travelled by bus in the Middle East it won't really surprise you, though the environment itself at the bus station is pretty horrible, Retiro is kind of like the central bus terminal in Istanbul) -- take a combi -- a 6-12 passenger van that will pick you up at your doorstep and drop you off at your hotel on the other end -- the smaller the combi the better as it will mean less travel time since they will have less stops on either end. You can see prices here:
http://www.lomasexpr...mos-a-la-costa/