Copenhagen for three days

I imagine the teens would love the chance to row a Viking ship at the Roskilde ships museum. They might even like the museum itself. Roskilde is a short train ride from Copenhagen and makes a great daytrip.

https://www.vikingeskibsmuseet.dk/en/visit-the-museum/activities/boat-trips/daily-summer-sailing-trips

The Viking museum is pretty interesting and the kids should enjoy that. And if you're going there, you must stop at Roskilde Cathedral, which is the burial place of many many Danish kings and queens. It's a gorgeous place and there are over 20 tombs in the cathedral. One of the crypts off to the side had eight tombs in it. Four of them contained the remains of more kings and queens, but there were four small ones that contained children. Three of them were for children who are age 4, six and eight, but the smallest one was for an unnamed infant. It was built in the 12th century and is well worth a visit.

Head to Roskilde Cathedral first when you get off the train in town, and then you can just walk down to the Viking museum from there. It's a very short walk through some beautiful greenery and takes you right down to the sea where the museum is.

Also, you must see Tivoli Gardens, which was wondrous! It’s a lot of fun just walking and seeing people eating and chatting and riding rides. And it wasn’t just family and kids there. There were lots of couples and groups of older people who were just walking around and enjoying themselves. It’s a place for everyone. If you can get there in the late afternoon, I would suggest you eat dinner there and stay till after dusk. The lights turn the place into magic. If you want some idea of what it's like, here is a link to my online journal with some descriptions and photos of Tivoli. I loved it there! https://mostlytraveled.wordpress.com/2022/05/21/day-25-pt-2-the-magic-of-tivoli-gardens/

Tivoli is a great place to have dinner.

Do they still have fireworks late at night?

Also visit "Bakken" - the worlds oldest amusement park: https://www.bakken.dk/english/

Tivoli is fine - we go there several times each year, but it is only the second (or third) oldest.