My best ones are:
Side Initiative: PCs roll individual initiative, but monsters roll as a group. So in effect, after round 1 it alternates between PCs and Monsters, and the PCs can go in any order they want.
Multiple Object Interactions: You can sheath two swords, draw a bow, kick open a door, and shoot someone all in one turn. I don't want to waste time saying, "Sorry, you've juggled too many objects this turn, you can attack next round." Players love this rule.
Stacking Inspiration: You can have multiple Inspirations, but when you take a long rest, you lose all but 1 of your Inspirations.
Inspiration Re-Roll: You can spend Inspiration to gain advantage after rolling a normal d20 roll.
Inspiration Consolation Prize: If spend Inspiration and both of the d20 rolls are 9 or less, add them together to get your roll result.
Critical Ability Check Results: If you get a natural 20 on an ability check, something special happens, at the DM's discretion. Generally, you succeed to the maximum extent possible, or get an added bonus beyond what you were attempting. When you add your proficiency bonus to the ability check, increase the critical range to 19-20.
Knocking a Creature Out: If you are using a slashing or piercing weapon, you must declare your intent to knock out before you attack, and if you roll a natural 20, the damage is lethal. (Very first session of Princes of the Apocalypse, the bard accidentally beheads the guy she was trying to capture alive for interrogation. It was darkly comedic.)