The other option rather than going 5-6 hours to the coast is to go to one of the clubs in San Isidro / Tigre for the day. Here's an article with some, there's also a bunch of clubs in Acassuso / Peru Beach where windsurfers hang out. Be aware that any of the prices in the links below may be far out of date (ie if it's from 2010 triple it at least, 2012, double it... unless it's in USD).
http://www.glamout.com/notas/146382-escapadas-pileta-metros-bsas
http://www.revistaohlala.com/1663513-opciones-para-pasar-el-dia-en-una-pileta-fuera-de-la-ciudad
http://peru-beach.com.ar/sitio/
http://www.glamout.com/notas/146382-escapadas-pileta-metros-bsas
http://www.revistaohlala.com/1663513-opciones-para-pasar-el-dia-en-una-pileta-fuera-de-la-ciudad
http://peru-beach.com.ar/sitio/