One of my favourites, when living in South Wales, was to walk the Roman road from the Mountain centre above Libanus.
The Roman Road winds its way all the way from Brecon Gaer fort, which is on the Builth side of Brecon, and makes for Coelbren, not far from Craig Y Nos.
The walk can be as long or as short as you want to make it, but it goes through some of the remotest regions of the park, traversing through Forest Fawr, and on towards Coelbren and beyond.
Its very easy walking, and has stunning views towards the main summits, and receives very few visitors.
I never managed to walk its entire length, as its route from outside Brecon leading to the Mountain centre, has never fully been verified by archaeologists, but the sections approaching the Mountain Centre from the Brecon side, are incredible.
As you drive up the winding road from the A470 from Libanus to the Mountain Centre, the Roman road is visible on the mountainside just opposite the entrance to the MC.
You can follow it clearly for a significant distance, and its only on the other side of Forest Fawr near the famous Maen Madog Roman monolith, that it gets tricky to follow.
The section near Colbren is quite impressive.
Its also one of those walks that can be combined by using your own transport, parking at opposite ends, to appreciate the wilder regions of the National Park.