hola oxente,
there is one that i have been to a few times - it's in devoto. the contact person there is is Hernan:
info@cpogym.com.ar
i originally asked about 'adult' classes (i'm too fat and old to keep up with the little 'uns) and i was welcomed to join in with the oldest class they have there, i bunch of lovely girls who do some standard gym stuff but mostly group dance routines. generally i would warm up with them and then do whatever else i wanted. they never asked me to pay a fee...
it's supposedly one of the better ones in terms of facilities.
suerte