Copenhagen for three days

You can take the train to Malmø in Sweden if you want to visit another country nearby, but I fail to see the point. Is it just because you want to say you have been to another country?
Malmø is nice enough, but there are other day trip options that are more interesting, like Roskilde, Helsingør or Hillerød. And with only three days, you barely have enoigh time for Copenhagen, let alone another country.

I agree with Morten. We had a week in Copenhagen and didn't go to Malmo because there were enough things in Copenhagen and on the island of Zealand to do that were much more interesting than just going across to Malmo simply to say we went to another country.

Thanks, so I would appreciate things to see and do. I am not a big fan of museums. Also how about Stockholm? any ideas on what to do?

Any guidebook and this website as well will give many suggestions on what to see and do. Start there and come back if you have more specific questions.

If you want to head to Stockholm, I think you'd be better off flying, but the train is an option. That's definitely not a day trip.

As Morten asked, what are you interests? Popular day trips from Copenhagen include (but are certainly not limited to) Hillerod with Frederiksborg Castle, Helsingor (Kronborg Castle, maritime museum, nice coastal town), Roskilde (UNESCO World Heritage site cathedral, excellent viking ship museum), and Mons Klint (stunning cliffs).

We will be taking a cruise. we will arrive in Copenahgen. Visit for three days. take the cruise, then visit in sweeden for three days.