Chell's mindscape resembles that place she first awoke after her stay in stasis: Aperture Science. Areas vary from the testing chambers to the observation rooms above, to secret cubbyholes, to the impressive lounge of the 1950s Aperture Science. The testing facilities are dirty and overrun with vegetation while the observation areas are clean and sterile looking. The cubbyhole areas are filled with writing and pictures that pertain to the people she's met in Niflheim and what they've taught her. In the lounge, a row of pedestals run down the center with artifacts behind glass museum cases. They include a bottle of alcohol, various weapons, a mini drum kit, several mini figures doing yoga, several mini figures dancing, etc. These are her most precious memories of new experiences that she stores here.
There are no murderous robots here and, in fact, the place is devoid of anyone, even Chell herself. Through reading her writing on the walls or looking at the artifacts, others can gain insight into what interests Chell and what she values. Due to the nature of her mindscape being empty of a guide, locking this to one character would be best, because I would be writing out scenery descriptions instead of interactions with your character and yours could come to their own conclusions about who Chell is based on what they see.