There are many, many routes you could do. Look at the routes on the site near to the location you are in and choose small sections of them. For example cycle 15-20km and turn around or find a road that can loop back to the start.

If you’re after flat terrain. Head to the fjords and cycle the roads along them. Lustrafjord, Hardanger, Hjørundfjorden, and Sognefjorden all have great roads to cycle on.

For example:
Choose a section of Hardanger: https://ridewithgps.com/routes/42464973?privacy_code=vOhmcNV1Hc2SlVJX
Hjørundfjorden: https://www.komoot.com/tour/1304995140
Lustrafjord: https://www.komoot.com/tour/1304996474

Check out my top five fjord roads here: https://youtu.be/SmgqzdDWU4E

Other examples:
1) Gamle Strynefjellsvegen is an amazing 27km (54km both ways), but it will close in the next few weeks
https://ridewithgps.com/routes/41403109?privacy_code=jqwJuos1XHltCJRU
When it closes it will say here: https://www.vegvesen.no/trafikkinformasjon/reiseinformasjon/fjelloverganger/#/gamlestrynefjellsvegen

2) Do the flat part of the Telemark Canal: https://ridewithgps.com/routes/42482340?privacy_code=iiers0xtb2d61RC2

3) Do the flat section of Tour de Dovre: https://ridewithgps.com/routes/39611333?privacy_code=xazexPNZobuJfiwv

4) Small Island loop from Molde: https://ridewithgps.com/routes/42506637

5) The islands East of Alesund are flat and nice to cycle (make a loop from parts of this bigger route): https://ridewithgps.com/routes/42506648

6) Small Røros loop: https://www.komoot.com/tour/1305014727