Inserting a "Dream Scenario" One-Shot in LMoP?

Running LMoP with a group of newbies and last session we stopped right beforing engagin with the Cragmaw Hideout. We have a session scheduled and a player notified me she won't make it and, after a few tries to reschedule, I decided to keep the session.

I'm thinking of running a parallel One-Shot with the existing PC's so the present players can get a bit used to dnd and its mechanics. Maybe a collective dream-like experience where, next session, they wake up back in front of Cragmaw Hideout?

Any suggestions for One-Shots? I was looking at Horror at Havel's Cross, just changing the starting point of it....