I’m looking for ideas on (Small-Medium) sized Halloween themed lobbies. It’s been an hour and I cannot think of anything new.
Any ideas help, but I’m asking that you don’t post “graveyard”, as I’ve already completed that. If you manage to help me, I’ll credit you in the experience, as a special ‘thanks’! 
How about the lobby is a haunted house! You could make some cool jump-scares and easter eggs in the lobby so players can have fun and explore between rounds.
As @xander5610 said haunted house is usually a must in a Halloween lobby, you could also add Halloween Bunting (Spiders, sweets, etc), a pumpkin with a decent size hole on the top full of sweets, Autumn trees as they look quite spooky in a dark environment, candy corn trees, Halloween lights with the colours orange, yellow and white as they are the Halloween colours.
Hope this helps!