The 35-mile-long Monmouthshire & Brecon Canal in Wales winds through the Brecon Beacons National Park, skirting past ruined castles, breathtaking mountains and picturesque villages. With only six locks, it's popular with beginning narrowboaters, but the excellent towpath also attracts hikers and cyclists and the water is enjoyed by canoeists and paddleboarders as well. Despite its many attractions, however, it remains "a popular place to be alone."