Aulne Abbey is a ruined Cistercian abbey on the Sambre, a few kilometres east of Thuin. It was founded in the early Middle Ages, rebuilt into a wealthy monastic complex, and then left roofless after the French Revolutionary period.
The surviving walls are large enough to feel like a real ruin rather than a token remnant. Pair it with the ASVi tram or a river walk if you want to stretch Thuin into a full day.