Li Shenron
Legend
I've always liked the idea that the first session be really simple: PCs meet, perhaps travel together, are attacked, investigate why they were attacked. Session over, level gained. It is like getting your feet wet, and past that awkward and horrible phase of having single digit HP (for many of them).
This could be as simple as:
"After leaving your home, you got employment as a caravan guard on the way to X." Caravan gets attacked by goblins/orcs/gnolls/undead, someone gets stolen away - children and/or young women to be sold to duergar slavers? Etc. PCs, all guards, rally to fight them off, then track them back to their lair, maybe face more of them and their leader, perhaps a bit of treasure and/or save a captive or two and/or a reward from the grateful merchant, whose daughter you saved.
It is a very simple scenario, can be played in one night, and doesn't require much planning - just decide what will attack them, who gets taken, what the lair looks like, if there is any treasure.
After that, level them up, and then they can go off on their first true adventure - a dungeon, or whatever.
But that wasn't your question, I realize. I was riffing off your desire for a one-shot adventure to get things started.
Yeah it's a bit late. I have ONE minute to leave the house
