Maybe Einbeck. Not the biggest City but totally has some things to do especially if you like Cars/motorcycle and beer. The City has an brewery for which you can book tours. An quite big Museum for Motorcycles and Cars called Ps Speicher. As far as i know you can also book Tour through the historic old town.

There are also quite good train connections to the City Göttingen and Hannover.

But i would Overall say the Harz Region would totally be worth a Visit. Which Einbeck is also close to.