I am going to be running a few one-shot sessions of 5th edition D&D at a local game store, for International Tabletop Day. I've had plenty of experience running long term games, but not so much with shorter one-shots and the like.

I want to avoid the usual dull dungeon delves that usually appear as part of mini-adventures, and want to toss people into something exciting, but not necessarily difficult to handle rules wise, especially for new players.

Any suggestions? If you were to be playing in a single session fantasy RPG, what would you think is fun?

Coming up with the mechanical side of things is no trouble. Looking more for story and scene ideas that would be unique and stand out.

Something "7th Guest"-ish.

Take the various "Murder-Mystery" Dinner Parties, games and other activities; and make an adventure out of it.

I wouldn't mind it including PvP concepts.  I wouldn't mind DMNPCs.  All of it could be fun.

PCs get gathered together, and are given a goal.  They could be some slightly obvious goals, but also some slightly less obvious.  In One-off adventures, you can even pull off games where each player has a different goal.  Because the commitment time is short: it's not so difficult for players and PCs to have interlocking goals without too much fear of losing someone.  Definitely not as appropriate for straight level-1 characters (better for level 3+.)  But, can be a boatload of fun nonetheless.

The party discovers a plot to assassinate a very well-loved king and has to decide how (or whether) to stop it.
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The head of a local monastery hires them to track down a stolen item, which leads them into a seedy part of town and dealing with the less-savory part of society.  They have to decide whether to steal it back, buy it back, or take it by force of arms (or acquire it via some other means...creative options should be encouraged).
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A small village hires them to deal with the leader of a roving group of bandits that keeps collecting 'protection money' from the villagers (a la The Magnificent Seven/Seven Samurai/similar movies).
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One of the party is accused of being a thief/murderer/whatever, and the group must prove his innocence in order to avoid being thrown into the dungeon as co-conspirators (naturally, this means avoiding local authorities in the process, as they don't care what the party is trying to do, they think the party is trying to escape and the only way to stop them is to lock them up.)
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Depending on the party, do a fantasy twist on the basic premise of any heist movie (Ocean's 11, The Italian Job, et al).  They all follow a fairly similar formula...

There's nothing wrong with "typical" fare, sometimes a normal game is what new players are looking for.

If you want unique, one-shots tend to be good venues for "cool ideas" you've had that would be very difficult to make a campaign around. Things like "evil" parties, the PC monsters defending against the NPC adventurers, escape from the Insert_Thing_To_Escape_From, catch/stop/defeat the comically over-the-top villain with cartoonish plan, etc.

I staged a fight for gladiators in an Arena game a while back, and the fight consisted of three phases:

Phase 1, they fought the crew of an Airship for control of the ship.
Phase 2, they got on the Airship and escaped, then fought the magically re-animated corpses of the crew a second time to firmly establish control.
Phase 3, the airship was crippled by a wood-eating demon, and they fought the beast while the airship plummeted towards the ground.  The players could make Acrobatics checks to move around in mid-air like they were flying.

The whole thing ended when the players used the corpse of the demon to cushion the impact with the ground, (barely) surviving to tell the tale.

I think there's something to this. If I play a demo, one-shot, or Con event what I want is a session of whatever is typical for the game or setting I am playing.

The thing I see done wrong with one shot sets is trying to focus on a task or situation. If you want it all done in a few hours it takes an art to focus people.

I have done a colasped mine, due to a subversive group. Their original exit blocked the must deal with those that feel they know too much. It was well enjoyed.

I also have done hunts for rare creatures. Gives you free reign to bring in any hostile creature and wondurous terrain.

PS having read the airship arena fight it was awesome.

that sounds like it could be a good one shot, with the multiple stage airship thing.

Maybe start the game off by giving each player a question to answer on a note card, something like "there is a damsel in distress, but the 'evil count' is offering you a bunch of money to leave and forget about it."

"You are designate the heir to a fortune, but must marry someone you don't know to inherit, do you?"

"You've been exploring what you thought was an abandoned castle looking for loot when you come upon a trio in a tense situation in the throne room, what do?"

"You were exploring what you thought was an abandoned castle looking for books on arcane lore. From how the dust is disturbed, it seems someone else got here a little sooner than you and has made off with the book. You can follow their footprints in the dust, but do you?"

And, if the players say yes to these various situations, you wind up with a scenario where the party face winds up married to an evil count, the party fighter takes a bribe to not interfere, the party rogue sees it happen (and probably tries to get in on that action) and the party wizard may wind up trying to bash the party rogue over the head for said book. If they say no, then things don't fall apart, because the party face now has a reason to fight off the count, the fighter will back them up, the rogue will be trying to sneak off, and the wizard will stay out of it.

The more interesting results would come up with mixes of yes's and no's, where the party face will say yes to the marriage, and the party fighter will try to rescue them anyway, while the party rogue and the wizard will show down over the book off to the side or something.