At any point during the game, type Ctrl+Alt+C, you should hear the sound of a door opening. Then type any of the following:

NOW THE RAIN HAS GONE - shows you the entire map
DO NOT FEAR THE REAPER - skips the current level
THIS IS MY CHURCH - gives you access to all rooms
FIT THE BEST - gives you access to all rooms and traps
I BELIEVE ITS MAGIC - gives you all magic spells
FEEL THE POWER - increases all of the monsters' skills
SHOW ME THE MONEY - gives you extra gold

To load any level, start dk2.exe with the parameter -level [level name]. The levels are in the DK2DIR\DATA\EDITOR\MAPS folder.