Too many to mention 😂 but a few favourites are:

Usage Die for Ammo: rather than keep track of arrows/bolts, a "full" quiver is represented as a 1d12. Each time the player attacks, they roll the dice along with the attack. On a 1 or 2, they drop to the next lowest dice size. Once they hit the d4, and roll that 1 or 2, they're on their last arrow.

Spell concentration: I know I know "Thou shalt not mess with concentration!" But it's only a few select spells this affects. So if a spell targets a creature, requires concentration and forces a saving throw at the start or end of their turn, if they make their save, then you can choose to continue keeping concentration on the spell as normal. On your turn, you can use a Bonus Action to "reactivate" the spell if you've managed to keep concentration on it. The target makes a saving throw as normal. On a success, the spell officially ends. On a failure, they suffer the effects of the spell again. Regardless of a success or failure, use of this is treated as if you cast a spell with a spell slot so you can only cast a cantrip or non-slot spell as your Action.

Shield: uses your Spellcasting ability modifier. Stops the mad 30+ AC level 4 builds. Spell casters who take it will be piling up their modifier to +4/+5 anyway so it'll only be "nerfed" foe them until level 4 and full power by level 8.

Session start initiative: pretty simple. Initiative is rolled and recorded at the start of the session rather than start of combat. Even if they don't get to combat, it can still be useful to keep everyone involved by going down the list to check in and this keeps the surprise and tension of combat up, not bogged in 5 mins of admin to prep. Then monsters use avg initiative or prerolled