Mar del Plata
Mar del Plata is a city in Argentina.
North towards Buenos Aires
Take bus #542 (in Av. Pedro Luro, or next to the main Casino) to "Aguasol" which is a waterpark some 5-10 km out of the city. You can decide either to stay until the last stop and enjoy the around 1h trip through nice villages or get off as the bus take the Ruta 2, from the airport on, around 30min after departing from the centre. It may be hard to get a ride but there's plenty of car going that direction and you should definitely find one.
South towards Bahia Blanca
Take a bus #720 to Batan, it passes through the city centre close to the Cathedral and a ticket costs 6 peso (~ 0.5 USD, June 2015), go untill its last stop and right there is a good hitchhiking spot.