4 nights in Copenhagen, or 2 nights somewhere else?

You can without problem spend 4 nights in Copenhagen, but going somewhere else is also not a bad idea. Møns Klint is an option, but there are others as well. Where in Germany are you going afterwards?

We are flying to Berlin.

Hop on the train to Helsingor and visit Kronborg castle and the maritime museum. Plenty for young teens to do at the castle - the almost pitch black casements tour is fun. It's an easy day trip from Copenhagen.

Do another day trip to Roskilde and go sailing with the Viking Ship Museum. With only four nights in Denmark, I'd stick with staying only in Copenhagen and making a couple of day trips. You can find plenty to do without museums.

I agree. Stay put in Copenhagen. Four nights is only three full days, and (perhaps) parts of two travel days. There is way more than enough to do in Copenhagen and surrounds to keep you occupied and interested.

(And yes, definitely sign up for the one-hour sail at Roskilde.)

Thank you for the ideas! Would you recommend taking the ferry over to Helsingborg, or not? Definitely sounds like a doable day trip and fun for the kids.

If you are going to Berlin, visiting Møns klint is really easy. Take the train from Copenhagen to Vordingborg, rent a car and drive to Møns klint and explore the area. Spend the night in Vordingborg and continue to Berlin the next day, but not by air. Take the train and bus to Gedser and the ferry to Rostock where you catch a train to Berlin.

I spent five nights in Copenhagen a few years ago with my son, then 10 years old. There is plenty to do for 4 nights. In addition to the aforementioned Roskilde and Helsingor, here are some things we enjoyed: Tivoli gardens, the aquarium, the zoo, a canal and harbor cruise.

Would you recommend taking the ferry over to Helsingborg, or not?

Not. Unless you are desperate for a ferry ride. Helsingborg is not that interesting and the ferries are more geared up for people with cars.

On our ferry between Sweden and Denmark in September 2022 most people seemed to be making the trip in order to buy copious amounts of duty free alcohol. People had the kind of trolley you use to move furniture stacked high with slabs of beer and cheap Australian wine.

Thanks ya'll for your suggestions. Sounds like we will stick with Copenhagen and have plenty to keep us occupied.

On the contrary, Helsinborg is an interesting town if you know where to look, and well worth a visit. The ferries do take cars but are there for all who want to get to the other side of strait. It is a rather quick trip with very frequent departures.