I'm sending my players to a demiplane of madness inspired by Xoriat. Tips?

[genderlich]

I can't send them to the real Xoriat for plot and balance reasons (for example, I'd like to have gravity). So instead they're forcibly transported onto a demiplane that has been built to resemble the Plane of Madness as much as possible.

I don't want it to just be constantly fighting aberrations. I want to trick them, fool them, make them question their senses and even their thoughts while keeping it combat-minimal. But I'm finding it difficult to come up with creative ways of doing so.

Right now I have them wake up on an island in an ocean of acid, guided by a mysterious voice (the BBEG) through a bunch of challenges until they meet him. (Partially inspired by the Ayreon album Into the Electric Castle if any of you are familiar with that.) But I have almost no ideas for the actual challenges. They shouldn't be overly lethal and should require creative thinking to solve.

The group is level 9; 3 or 4 PCs accompanied by 2 NPCs of equal level for plot reasons. I'm also thinking about bringing along two low-level NPCs to serve as cannon fodder. We're in Pathfinder if that matters.