Blessed, Blessed Is He Who Cometh from Saint Saens’ Christmas Oratorio is a beautiful duet, not too long, and not operatic. It was a great favourite at my conservatory, and I’m surprised it’s not performed more often.
You can make a superb harp part from the piano-vocal choir score, though I wouldn’t play it in as staccato a style as this pianist does – slightly arpeggiated chords on the harp work wonderfully.
https://www.youtube.com/watch?v=vC8xkg2j8O4