I would, simply to tell them they're doing great and many people appreciate their talents and work and keep on doing what they're doing, then tell them they can use the remaining time to just chill if they want to. They're people, not trained dogs, so I don't want them to do challenges or aegyo or sing or do anything like that unless they feel inspired to sing or dance or whatever because we're having a good time and they feel like it. (Like if I said a word that reminded them of a song lyric and they started singing to be silly). I'd leave it up to them what they want to talk about or not talk about if they'd just have a few minutes where someone's not expecting something from them.
If I had longer than a 2-5 minute call, then I'd love to chat about music - what music they're into, stuff I'd recommend if they like specific artists, their songwriting and craft and approach because I love music and talking about.