What is your definition of traditional pieces? If it is folksongs it would help to know where in Denmark, because some of the local popular ones is tied to lotations for example the Limfjord.
If it is very old peices of music it would be some other pieces an example could be the oldest known danish folksong “Drømte mig en drøm i nat” (try wikipedia for the melody line)
There is of course THE weddingwals (brudevalsen) in Denmark, which is the one the bride and groom dances the first dance to and normally not used at any other time. It is written by Niels W. Gade in 18something. I think I have the melody line somewhere on my laptop – send my a message. But maybe you should not play this one since it would be a shame to let you miss the fun on the dancing floor the guests are doing around the couple.
In genereal things are not arranged for harp, but many have piano arrangements exists besides the melody line only.
In the older generations where i live an old melody from Scotland called Should auld acquaintance be forgot is well known with danish lyrics by Jeppe Aakjær.
At last, maybe ask the couple about some melodies they have special feelings for.
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