The two enjoyed their stay there until Sylvia decided to leave while Wander wanted to stay, in which case a lot of trouble had to go in order for them to get out of the place.
The void itself is, as it name implies, white and featureless where the laws of physics do not exist, though it can be changed by the ones who visit it. This can range from minor things (Adding a few objects), to complete remake (Wander turning it in a studio with many Wanders during the Inside Your Mind musical number).
Outside the void is a doorknob and a brown door leading to it. However, because the Void makes any wish come true, multiple visitors have to be in agreement to leave the Void in order to use the exit door. If one part disagrees, any attempt to use the door will be distorted by the part's imagination.