Forty minutes on a direct train, no changes, no car needed. Liverpool asks nothing of recovering legs that a short walk or a taxi can't solve, which makes it the easiest overnight add-on on this list.
The Waterfront and Museums
The Royal Albert Dock puts the Merseyside Maritime Museum, the Tate Liverpool and the Beatles Story within a few minutes of each other on flat, level ground. All three suit a day of standing and sitting rather than walking, which is exactly what a body one day past a marathon wants.
The City Centre
Liverpool's two cathedrals, the Anglican and the Metropolitan, sit at either end of Hope Street, a walkable but not short route between them. Treat the full walk as optional and lean on buses or a taxi if the legs vote against it.
Where to Stay and Eat
Liverpool's city centre and the Baltic Triangle both offer a strong concentration of hotels and restaurants within a short taxi ride of Lime Street station, keeping the whole visit low on logistics.
Getting Back
Direct trains run frequently between Liverpool and Chester throughout the day, keeping the return leg as simple as the trip out.