"A Dark And Stormy Night": The PC's are in an Inn. They find it has been surrounded by something. The PC's must defend the Inn against attack, and deal with the fears, injuries, and drama of the other trapped patrons. Suggested attackers: Zombies, Sahuagin, Ettercaps, Mummies. Relatively low tech, low intelligence brute attackers with no love of fire are preffered because the goal is to make the Inn have tactical value. You can play this straight, or have as a twist the object of the attack be something in the Inn (some cursed treasure that another patron has stolen) and the attack will not cease until the treasure is restored to its rightful gaurdians.
"We're Not Drinking In Kansas": The Inn is no ordinary Inn, but some supernatural location and/or the PC's have accidently stumbled into some important meeting of some sort. Examples: the inn is cursed and whoever stays until morning must stay forever, the inn and everyone in it is actually a ghost, the inn is an illusion created by a powerful wizard or outsider with strange motives, the inn is actually a wandering location run by the God of Wine and Revelry - the other patrons are gods or similarly powerful beings, a group of powerful outsiders (slaad, fiends, modron) are in disguise and having a meeting in the inn, everyone in the inn is a werewolf, etc.
An inn can be a good oppurtunity to introduce NPC's that are otherwise way over the PC's heads and status at this time, but which may be important later in the PC's careers. This can be anything from having one of the patrons actually be the crown prince slumming, to the Innkeeper is a friend of the best swordsman in the world, to having five or six of the Slaad Lords from The Explicatae Incompositae (see link below) meeting in the Inn.
"Checking in to the Bates Inn and Tavern": The operator(s) of the inn are actually serial killers/cannibals/cultists, who regularly poison and/or and murder travellers in their beds. There sinister actions may be unknown to the locals (who are safe by the rule of don't dirty your own nest), or they may even be known and approved of by the larger community. The PC's must survive the night, uncover the sinister secret, and make the road safe for future travelers. This story is actually older than dirt, as it's one of the challenges faced by Theseus.
"The Murderer Must Still Be In This Room": The PC's are staying in a remote tavern when a murder occurs (typically, it's not only remote but a storm, blizzard, lack of boat, etc. prevents anyone leaving). With no law enforcement nearby, the Innkeeper shuts up the Inn and refuses to open it until the murderer is revealed. The murderer has planted evidence that implicates on the PC's. The PC's must solve the crime and reveal the real murderer. A nearly infinite number of twists are available: the murdered person is their own murderer, there is a doppleganger or other supernatural being among the guests, the murderer has reasons that the PC's might actually be sympathetic to, the innkeeper actually did it, a PC actually did do it but was under a compulsion, everyone or almost everyone but the PC's are actually in on it (in which case see "Checking into the Bates Inn and Tavern").