Reminder that the WSV (Wasserstraßen- und Schifffahrtsverwaltung des Bundes) offers free ferry rides across the NOK. Ideal for hikers and bikers, as there are only few bridges.
https://www.wsa-nord-ostsee-kanal.wsv.de/Webs/WSA/WSA-Nord-Ostsee-Kanal/DE/1_Wasserstrasse/2_Tunnel-Bruecken-Faehren/3_Faehren/1_Fahrplaene/Fahrplaene_node.html
The locations (pic related):
https://www.wsa-nord-ostsee-kanal.wsv.de/Webs/WSA/WSA-Nord-Ostsee-Kanal/DE/1_Wasserstrasse/2_Tunnel-Bruecken-Faehren/3_Faehren/3_welche_faehre_faehrt_wo/welche_faehre_faehrt_wo_node.html
I can also recommend taking the train to Rendsburg, as the view from the bridge is spectacular.