The Wye Valley Walk has one of its trailheads near the castle in Chepstow, while Offa's Dyke Path starts just over the river in Sedbury. For a taste of both, you can walk upriver to Tintern Abbey on the former, cross the bridge just past the abbey and take the path to Devil's Pulpit, where you can join the latter for the return leg. The total distance is around 13 miles; allow a full day, with lunch at Tintern. Ordnance Survey (OS) Explorer map OL14 is recommended. You can cut the walk short at Tintern and return to Chepstow (or continue to Monmouth) by bus.