The Monmouthshire & Brecon Canal runs along the side of the Usk Valley into the Brecon Beacons, and its towpath is gradually being upgraded to become a cycle route, NCN 49. It's mostly unpaved but smooth and firm.
The Beacons scenery is every bit as fine as you might expect, though much of the canal is tree-lined so you won't have a continuous view of the mountains. The towpath is cyclable all the way into Brecon, though it's not yet all been upgraded to NCN standard; a hybrid or gravel bike will be fine except in the wettest weather.
Following a busy holiday waterway through a National Park means that this route is well supplied for cafés, pubs and accommodation, making it particularly suitable for a family cycling short break.
Long-term ambitions are for the route to continue over to Llandeilo and Carmarthen, with work due to start shortly on the Tywi Valley Path, a new rail trail between those two towns. For now, you can ride to Llandeilo along the glorious lanes that form the Cycle across the Beacons route.