The Weserwall is the easy river walk on the edge of Nienburg's old centre. It is not wild countryside, but it gives the town air: water, trees, bikes, and a reminder that Nienburg belongs to the Weser as much as to the roads between Hannover and Bremen.
It is also the simplest way to fold Nienburg into a wider Weser trip. If you are cycling the river route, this is where the town makes the most sense.