Once you look at the tool menu in GMod, it gets easier. And making HL2 characters do silly stuff is actually pretty easy. Just take the physics gun (the default gun) and left click and hold one of the ragdoll's limbs. Then, move it anywhere you like, and right click to freeze it in that position.
In GMod, here are some things I made:
Crane w/ wrecking ball
Suicide booth
Hovercraft
Spaceship
Fusion bomb

It's easier once you get a few more mods (mostly wire, PHX, and Stargate)

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