The party is asked by their friendly neighborhood innkeeper to retrieve something for him from the attic of the 3 story inn.
variant 1) Into the Attic: After climbing stairs for more than several minutes, not seeing a top to the staircase nor the bottom "hatch to the attic" they were instructed to go up, they eventually realize that something is amiss...hours of stairs later, they come to a doorway that let's them out to...wherever. Pick or make an adventure and enjoy.
variant 2) The Fourth Floor: The party find a staircase on the 3rd floor that goes up and leads them to another (4th) floor full of room numbers. The inn, from the outside is only 3 floors, remember. The 4th floor is an extradimensional space with each "room" leading to any number of other places/planes. Maybe make it so you arrive in a room at an inn in whatever other place you go, so upon leaving the room, you're not back on that mystery 4th floor. You're in another inn in another town, or country, or plane...or time.
Achieving the adventure du jour somehow gives them some kind of dimensional gate that lets them back out...you guessed it, on the 4th floor. Only after retrieving the item the innkeeper originally sent them to get can the party find the stairs that will take them DOWN back into the "normal/real world" inn.
variant 3) Room Number 13: The party is in an inn (any floor) and hears a terrible commotion/someone in distress in one of the rooms. Being heroes, after all, they race into room number 13 to find themselves transported to...wherever. Pick or make an adventure and enjoy.
Completion of the adventure du jour somehow ends with the party returning to the inn, through the original room #13. When they make their way to the bar and encounter the innkeeper he will, of course, inform the party that the inn HAS no room numbered 13. "No inn ANYwhere has an room number 13. Bad luck, ya know."
--Steel Dragons
