So I just read about this and haven't had a chance to read Under Hill By Water yet. But right now I'm thinking about making some kind of simple location to explore with a number of interesting characters to meet. I'm thinking some kind of large party similar to Bilbo's birthday could work well.
I'm mostly taking inspiration from The Blancmange & Thistle from Troika. My favorite time playing that my players just had a great time meeting all the interesting characters.