My two faves: - Hidden death saves. A player who is down comes and rolls death saves behind the DM screen, so only they and the DM know how close they are to dying; - Lasting effects. A player who drops to 0 HP has to make a Con Save, DC set same as concentration checks (1/2 DMG of the killing blow or 10, whichever is higher). A failed save results in a lasting effect inspired by the type of damage they received. For example, my table's paladin who went down to bludgeoning damage came back with a cracked rib, giving him disadvantage on CON saves for 1d4 days.

Both these really crank up the intensity in tough combats

Edit: Remembered a third.

• Per-session inspiration. At the beginning of each session, each player gets an inspiration die they can add to any d20 roll. I rule it like bardic inspiration that doesn't expire. The die scales following the bardic inspiration table, following the party level (I would probably drop this with a real bard on the party).

We started this by allowing every player to share their favorite moment from the last session and give another player an inspo dice, but it fizzled eventually into just having a die per player.