Grenz-Dokumentationsstätte Lübeck-Schlutup is a secondary but useful sight in Lübeck, the sort of place that rounds out an itinerary once the obvious highlights are covered. It is a little away from the tight centre, but still close enough to fold into a half-day plan if the subject interests you.
It is worth checking current access before going and pairing it with nearby stops. Give it enough time to be more than a pin on the map; the smaller sights often work best when you arrive with curiosity rather than a strict schedule.