Level skip:
Lara must be facing north. Use the compass to verify that she is. Select "Load Game" and, input H, E, L, P. Now go to the inventory screen.

All weapons, infinite ammo, medipacks:
Lara must be facing north. Use the compass to verify that she is. Enter the inventory screen and select the "Small Medipack". Now input G, U, N, S, and then exit the inventory screen.

All items:
Lara must be facing north. Use the compass to verify that she is. Enter the inventory screen. select the "Large Medipack" input W, E, A, P, O, N, S, and then exit the inventory screen