Fallout
Lots of gameplay, fun.
Doom
You've probably already played it, but this version has awesome mods.
Wolfenstein 3D
Classic FPS, also mods.
Terraria
A little like a 2D minecraft, with big focus on adventure.
Garry's Mod
Pose characters, build structures, vehicles and almost any machine you can think of. Also has RP gamemodes, deathmatch, and many more original gamemodes.
Dino D-Day
World War 2, except you can also play as dinosaurs as well as humans.
or Gift your friend a game so he can play it with you
