Day 1 of the Super Brevet Scandinavia route fits the bill for flat
https://ridewithgps.com/events/6723-...703413/preview
It includes three ferry rides, which is pretty cool though it does add logistics. Depending on the daily miles you want to cover, the towns could provide lodging options.
I really enjoyed riding in Denmark. It's different than riding in the states. Cycling is a normal activity, accommodated with infrastructure, but also cyclists are expected to follow the rules just as rigidly as are motorists. The laws are a bit different.
I'd say I enjoyed Sweden a bit more - see the day 2 route from the above link - but it's not as flat.
Cheers