It would depend a whole lot on the person! I avoid lying whenever possible so I can’t just make up something nice to say either. But funnily enough, there is one thing I know about them! And this is actually a good one I’ve said before to some of what came to be my doch.

“You seem like someone I’d like to get closer to.”

In the context of prospective doch they’re usually young so just telling them that truth is way more forward than they’re used to getting, other than like, sexual advances. Even for an adult it’s rather intimate and surprising. If it’s not romantic interest the tone just changes a bit, not quite as effective there, but usually the best way to get someone interested in you is just to show genuine interest!