If you want a little extra bonus challenge, here's a set of pseudo-achievements to try and get!
(These are not acknowledged in-game)
(Contexts may include scenario spoilers)

Workplace Hazard - Kill the G-Man once. (On purpose, as bonus cred)
In Soviet Russia… - Get eaten by the vending machine.
Where's the Fire? - Speedrun efficiently enough to make G-Man teleport in front of you to keep up.
Get Back to Work - Locate the hidden ending in Train Station 02.
Late for Work - Clear Black Mesa East in under 7m:35s. (Starting from the moment the airboat drops until the final cut to black)
Post-Mortem Combustion - Blow up a Zombie with a barrel being fizzled by G-Man's stasis field.
Pour out the Spirits - Locate Barney's ghost.