If double damage is too swingy, I wouldn't mind criticals imposing special effects, for example:
Greatclub. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 18 (3d8 + 5) bludgeoning damage. Crit: 18 (3d8 + 5) bludgeoning damage and if the target is a creature, it must succeed on...