Proper Hiking close to Buenos Aires is going to be very tough.... the best place for good hiking is Tandil, and thats a 4.5 to 5 hour drive away.

If you are a serious hiker, then you need to consider going to Bariloche, San Martin de los Andes, or Mendoza. Some areas of Cordoba may also be of interest. El Calafate is also a good option for ice trecks.... this may also be doable at the Ski areas depending on when you go.