The ferry port at Cherbourg provides great access to bike routes in northern France. Here's one idea.
This cycle tour explores the beautiful Cotentin Peninsula and can be completed in anythign from 3 o4 4 days through to a week or more. The section to the east covers key World War II sites, including D-Day beaches.
This is a beautiful area that I cycled in 2016 – see here.
If you don't have time for the full route, you can take a short cut from Carentan to Haye-du-Puits to cut the distance in half.