@Andreas and I were just chatting and this idea to use gelatinous cubes as a mode of transportation came up. Essentially, you just hop into a cube as it slides down the narrow tube and it carries you, safely, to the next level below. The metal grate prevents you from moving on, but the cube slides right through. A portal loop sends the cubes back to the top to repeat the whole process.
An interesting alteration would be to remove the bottom grate, but the portal only transports slims. So you have an elevator down but getting back up is going to require another route.
Oooh, that is an interesting twist! I like it Especially if this is just a portion of a larger complex instead of the Elevator being the central shaft.
So inventive! Love it!
You could introduce this puzzle in say the first room of the dungeon to get the players acquainted with it and in subsequent rooms use the same mechanic but incorporate it into more difficult puzzles or combat encounters (or both!)