Peñol is a huge granite mega boulder located in Guatape about two hours outside of Medellin. There are not too many routes but it is a beautiful area full of island lakes.
From Medellin take the bus from Portal Norte two hours to Guatape and ask to be dropped off at La Roca. Once you are at the base of the rock walk around to the opposite side from the ticket office where you will find the routes.
The cheapest way to do it right now is to stay in a hostel in Guatape and take the bus in. However, there is a climbing hostel currently in the process of opening at the base of the rock but as of the time of this writing it is not open and has no name.