Level skip:
Have Lara face exactly north. Enter the inventory screen and look at the compass. If it is not transparent, try positioning her again. Go to "Load Game", hold H + E + L + P, then release those keys. Close the inventory screen to advance to the next level.
Unlimited items:
Have Lara face exactly north. Enter the inventory screen and look at the compass. If it is not transparent, try positioning her again. Go to the small medipak, hold G + U + N + S, then release those keys.

All weapons:
Enable the "Unlimited items" code. Close the inventory screen. Enter the inventory screen again. Go to the large medipak, hold W + E + A + P + O + N + S, then release those keys.

Hint - Easy way to face exactly north:
Find any block that faces north. Climb up or jump to it. Go to the inventory screen and your compass should be transparent to confirm she is facing exactly north.