"It was nice that you came to the opening."
Translation:Det var trevligt att du kom på invigningen.
”Fint” is more often used for nice-looking. You could use it here as well but they don’t mean entirely the same thing. ”Det var trevligt att du kom” means sort of ”we had a nice time together” whereas ”det var fint att du kom” means more ”it was a nice gesture that you came”.
I think that corresponds better to "It was nice of you ..." in English.