Which are the best companies to book day trips out of Copenhagen. Travelling in September 2025

Very, very, very easy to DIY.

Viking ships - Do you mean Roskilde???? - Train from Copenhagen, multiple time options, bus 203 to the Viking Ship Museum from Roskilde Station or pleasant walk through parkland. There are English language guided tours offered at the Viking ship museum. Also, all the info available in English.

Not sure why you are looking for a guide to take you to Sweden. Where did you want to go?

Easy train to Lund or Malmo from central Copenhagen station. OR - take a train up to Helsingor, Denmark. Visit the castle (Kronborg - model for Shakespeare's Elsinore) and/or the historic city center or Maritime Museum. Then walk over to the ferry and take the ferry to Helsingborg, Sweden. Wander for an hour or so - city center is right by the ferry port. Ferry ride is 20 minutes, pleasant craft with food, drink, and shopping options. Unless you need a car reservation, no need to purchase walk-on travel in advance.

The Copenhagen Card is one of the few tourist cards that really gives you great benefits including public transport to the areas I mentioned above.
https://www.copenhagencard.com/view/Transport

Ferry website:
https://www.forseaferries.com

Journey planner:
https://www.rejseplanen.dk/webapp/index.html?language=en_EN#!P|TP!histId|0!histKey|H634987

No experience with guided tours, but I suspect you can look at Viator and find some that might be to your liking. When it is so easy to do this myself, I hesitate to book tours that may not be to my liking, but that is my preference - nor necessarily yours!

Some inspiration for day trips around Copenhagen.

I agree with ORDtraveler, it's very easy to do on your own. Trying to pay someone to drive you instead will cost a lot more, and most likely take a lot more time.