Is Malmo worth a half day or so if you only had two full days in Copenghagen?
If the only time you have in the area is two days, don't consider going out of Copenhagen. Here are some suggestions of what to do:
Take a canal boat tour. Visit the National Museum. Walk along the Stroget, stopping at one or more of the squares for a coffee, beer, or icecream and people-watching. See the Rosenborg Palace. Visit Tivoli. Drop in at the Museum of the Danish Resistance.
If that doesn't fill up two days, you can go out to the Viking Ships museum at Roskilde or the Louisiana Museum of Modern Art, each a short distance outside the city.
I agree totally with Rosalyn. The other thing we saw in the Copenhagen area that we liked very much was Dragor.
I meant to mention one of place very worthwhile to see: Frederiksborg Castle(Danish Versaille).