Most Interesting travel between Copenhagen to Paris or London

Not sure what you mean exactly. Do you mean which places are most interesting to visit along the way? But how long do you have total? Will you be driving or on the train? Are you looking for fun things that will interest the 6 year old? Or do you just mean which route is most scenic from the train? The options are so varied and almost endless that it's hard to give much of an answer.

Are you trying to decide how to travel? Train, Car? More info please.

Yes, I'm not sure either. I'll be in Copenhagen for a few days on business and want to give my 6 year old a memorable travel experience. I'm sort of thinking a scenic train ride or road trip. Even staying at a castle or interesting town along the way would be in the cards. A ferry ride somewhat in the works might also be appropriate. (I had read a little about a train that actually goes on a ferry on part of its journey.) Any experiences that people have had doing a like trip – and would recommend – would be appreciated. Thanks

With a 6 year old you may want to consider Legoland in Billund Denmark. Not sure about your 6 year old, but mine is addicted to Legos. We visited the Legoland in CA and it is great, I'm sure the one in Denmark is excellent also.