If it were me (knowing what I know nowadays) I would honestly spend 1-day in Paris and 1-day in Brussels.

Both places are dumps. Really went downhill bad and can be quite dodgy. You can see the main bits of each in a day IMO. France and Belgium are both very well connected by train - you can travel about both countries at will, with ease and see lots of fantastic little towns and sights around both countries.

Mechelen in Belgium is a good one and is only about an hour from Brussels with fast and frequent trains. Brugges is another very nice place that won't take too long to get to, either.

France I can give you lots of recommendations for (depending on how long you want to travel by train).

As for Copenhagen, great choice. Again though, Copenhagen isn't a massive city you won't need more than a couple of days there. You should consider getting the train across the bridge to Malmo in Sweden (only about 1.5-hours) or if you're feeling really up for it get the train up to the top of Denmark to Aarhaus and Aalborg.

The scenery, way of life, architecture etc in Denmark is just fantastic. It's a beautiful country.