Play the version of Half-Life that never got to see the light of day.
From strafing grunts that are far more aggressive to telepathic kingpins that can toss the player around and teleport at will.
Grab hold of some new weapons to add to your arsenal and spice up the combat.
Take advantage of your inventory system to keep you going for longer. Find your way through new areas that have never been released, or have made it off the concept art.