Hamelschenburg is one of the finest Weser Renaissance castles near Hameln, with ornate gables, a moat, formal courtyards, and a remarkably complete ensemble of outbuildings and church. It feels more aristocratic and rural than Hameln's Pied Piper old town.
The castle is a good half-day excursion if you are exploring the Weser hills by car or bike. Check tour times before going, because the interiors are not simply open like a public museum.