If you can avoid the hot hours by visiting this place early in the morning, I recommend the Philopappou hill, which resides in the center of Athens, next to Acropolis. There is a free parking area, outside of the "Dora Stratou Theater", only 200m from the rocks, but you can also go there by taxi.
There is a guidebook (by D. Titopoulos). Unfortunately it is not online, only on hard copy. Usually there are many climbers in this area, if the day is not very hot. The most common boulders is the "Traverse" and "Potamousis rock" with passages from V+ to VIII+.