Change the set date to December 25th and all pine trees will be decorated and have presents under them.

Hold LEFT SHIFT and KEYPAD MINUS simultaneously on the main menu or during gameplay, release them, and enter these cheats:

campaign - All missions unlocked
topography - Show map elevations

Hold LEFT SHIFT and KEYPAD MINUS while playing, release, and enter these cheats:

iwillbetheone - God mode
savegame - Creates saved state
endmission - Ends current mission