I have a bunch of houserules in my game, but here are some of my favorite and least complex:
Sprint: If you do not do anything else on your turn and you are not in difficult terrain, you can move up to 5x your speed (150ft typically). Attacks of opportunity against you are made at advantage. This is mainly to allow characters to catch up to combat without waiting 10 rounds.
Fight or Flight: Replaces the frightened condition. You can choose to flee or fight. Fleeing is unambiguous, fighting entails doing everything you can to kill the source of your fear -- no healing, no hiding, no stabilizing, no keeping your smite slots for later. Failing a save by 5 or more forces you to flee. (Taken from an XP to Level 3 video.)
Death saves are rolled in secret.
Light weapon property: we use the OneD&D version.
Critical hits: If you kill a target with one, the damage spills over to an enemy of your choice if I deem it to be within range of that attack. The damage keeps spilling over as long as you kill enemies. For instance, a critical hit with a bow worth 35 points of damage could kill up to five 7HP goblins if they are conga-lining in your direction.