The idea of having a material that changes properties based on what you convince it to have it amazingly interesting.
There was also the portal to the afterlife that someone dragged a boat, of all things, through. It was a kind of demi-dimension that was nothing but red pseudo-water in every direction.
Could make an interesting plane. One with nothing but strange and breathable water in every direction, people drifting and swimming through it.
There was some other things, but it's been awhile..
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather