There are a number of areas in the game that can only be accessed through the use of the Console. To access these hidden areas, open the Console by pressing the "~" then enter one of these commands:

coc toddtest
coc "character stuff wonderland"
coc "clutter warehouse - everything must go!"
coc "ken's test hole"
coc "Mark's Vampire Test Cell"
coc "Redoran interior"
coc "Redoran interior2"
coc "Draugr Test" (Bloodmoon)
coc "Mark's Script Testing Cell" (Bloodmoon)

Credit: Gamespot