The strongest (impression wise) was always "different wall/floor texture" to me.

I already use this to some degree, but I should utilize it more. From the camera view the floor is most prominent.

- stuffed dragon

Lol, sounds funny. I could think about some really giant object/statues. The level is kind of cut open and a really large statue, a stuffed draon, which towers above the rest of the level should be an eye catcher

Some larger, towering objects like

- stuffed dragon

- statue of a god

- pyramid

Some larger, towering objects like

...

A few more suggestions for such:

Castle:

- A ridiculously gem-encrusted gold statue of the king, in a theoretically-heroic pose

- A menacing statue of the villain

- An obelisk, perhaps a monument of some sort

Sewer:

- A sludge-fall

- A tall arrangement of dingy brass- or iron- pipes, likely some part of the sewage works

- A shaft of bright light from some opening above

Caves:

- A waterfall

- A limestone column

- A lavafall

- A huge, ancient idol