Gain one of the following
Hearty - Heal equal to your con per turn.
Sturdy - When at full health and getting hit by an attack that would reduce you to death saving throws, instead survive on 1 health. An attack that would cause instant death bypasses this.
Pure Body - Resistance to one status of your choice
Iron Skin - Resistance to Blunt, Slashing and Piercing dmg, -5 dodge
My Sturdiest Shield: Create a link between your Constitution and an ability of your choice. This changes both the ability and you