Hamburg
Hamburg (like Lubeck) has a long and proudly independent history as free city that had been part of the Hanseatic League (HH on the number plates refers to Hansa Hamburg).
There's loads to see and do, or if you prefer, you can just do nothing and pass the time loafing around the string of parks from Planten und Blumen down to the Elbe or stroll around the Alster.
Architecture is a big thing here, with the Chilehaus among the many highlights.
Hamburg Greeters
If you want to be shown around the city by a local, why not book a greeter for free? Details can be found here: https://www.hamburg-greeter.de/
Station strolls
If you are just changing trains at Hamburg and fancy a leg stretch, here are some ideas. I've also put a couple of things near Hamburg Altona as well.
Bike hire and other low-impact transport
https://www.hamburg-travel.com/discover-hamburg/experience-sustainability/low-impact-transport/
Further details:
Tourist info details can be found at: https://www.hamburg-travel.com/
and for their sustainable travel initiative: https://www.hamburg-travel.com/discover-hamburg/experience-sustainability/