>> How about, as another alternative, 'ek hefi skemtan af náttförum'?

Sounds good, - the only problem being the ambiguity. 'Náttförum'
can also be dat pl of "náttfari" (nightwalker), so the sentence
could mean "I am amused by nightwalkers".

>> I wonder if *náttgöngull would be understood in the same way

Hmmm - I can't think of an objection, but somehow it doesn't sound
as good as "náttförull". Possibly it brings to mind "svefngengill",
and veers towards "sleep-walking" in the reader's mind? Not sure.

Cheers,
E.