It's been a few years now since I've visited, and a few more since I lived there for a couple of years, but Schleswig-Holstein was my "German home", so I certainly know how things used to be.
Agreed the Rendsburger Hochbrücke is a great thing to travel over - although I'm not sure what the view is like at the moment as I believe they are performing some sort of works on it again (or between there and Felde). I haven't been over it since the new double-deck trains were introduced either, which may give an even better view. Travelling northbound, the line then loops around the town, and the station is almost underneath the bridge. (Was quite handy for knowing whether you'd make the train or not... I could just get to the station in the 8 minutes it took the train to loop).
There is also a transport bridge under the bridge, which is now back in service after bashing a ship some years ago - if the cafe is still open, it plays the national anthem of the passing ship. Not super-close to the town centre, but worth it if you like quirks.
There are new battery trains on the Neumünster - Bad Oldesloe RB82, with the RB63 to Büsüm to follow. They meet swiss-style in Neumünster. More battery trains should be introduced soon (if not already) as part of the plan to de-diesel the network.
There's a beach at Eckernförde if you get bored of fields and wind turbines (random aside: I recommend the hand-made sweets from
https://www.bonbonkocherei.de/)
Fully agree with those who say the Hindenburgdamm is interesting in its own right - the only way onto Sylt, and it's not road! At Niebüll, there are various connections with bits of the NEG. The service to Tønder in Denmark is now a boring Arriva Danmark LINT (I think!), but the neg look to still have something elderly on their own journey to Dagebüll. I never did it myself, but I understand it goes right down to the ferry terminal to Föhr and Amrum (see
https://upload.wikimedia.org/wikipedia/commons/e/e4/13-09-29-nordfriesisches-wattenmeer-RalfR-03.jpg).
"Sadly", the VTA diesel units from AKN seem to have been replaced by more LINTs, although the southern section between Kaltenkirchen and Hamburg is being converted to S-Bahn operation.
Not the most exciting (and relying on it actually working...), the three-part folding footbridge in Kiel is an architectural wierdity:
https://en.wikipedia.org/wiki/Hörn_Bridge
There's a pretty decent map at
https://www.nah.sh/assets/05-Karten/BahnlinienSH_A3_2024_5.pdf, and with a Schleswig-Holstein ticket you should be able to plot yourself a circular route... and still have time to go to Miniatur Wunderland!