Think I would go with D&D like you said, it's probably the only boardgame that's different each time you play, I mean Monopoly and Chess seem like obvious choices but monopoly would get boring very fast and short of using the top hat to fend of fashion fearful attackers it's not much good once the boredom sets in, at least Chess would keep your mind occupied and to a larger extent D&D would keep both your mind and imagination active.