Optional Advancement System
Taken from a three-part Sci-fantasy funnel I'm writing:
This adventure offers an optional experience method written in the spirit of a quick game with random characters. When your PCs survive the first combat or perform any major action that supports the story and their motivations, they go to 1st level and choose a class.
Thereafter, leveling up means surviving one combat or task greater than your character’s current level: a 1st level PC needs to survive two combats or perform two tasks to get to 2nd, then three to get to 3rd, and so on. This way, your PCs level up fast and future encounters can match their advancement as the adventure progresses. You can justify training through intuitive leaps of logic, adding a few synchronicities for good measure. This option is far from perfect, but it’s expedient for a quick-paced game.
Explain this method to your players and have them write a list of accomplishments and actions for their characters as they go. This style encourages role-play and participation, with players on the lookout for their moment. They want the spotlight, so give it to them. You can use these exciting touchstones to recount the tale later.