You could also go with the good old fashioned pit trap.

Perhaps combine all the traps and make a spiked, magnetic, poisoned, radioactive pit, with the toastantor and a gelatinous cube, and then a sprinkling of sodium, blast of brine with a huge pouring of oil of slipperyness. Fire half a ton of crossbow bolts, and a incendiary cloud, you are done with that party

Overkill 101 is that

If you like gelatinous cubes then you'll like the ice cube trap.

Basically it looks like a puzzle - there is a hole in the ceiling or a pivot or something, and lots of 1m cubes of ice in the room, stack them up or position them correctly to get out of the room. But, the cubes are actually frozen gelatinous cubes - and they are thawing out!

Hehe, perfect...

You may also like my evil creation the Hydratinous Cube, which is specifically focussed on facing higher-level players who won't just run from it!

Hmm, I would change up that splatter ability for a stupidly high level group to 2d20 damage per turn (re rolled every turn), 1d20 little guys, and 1d20 hp per cube

Then again I enjoy scrubbing players of the face of the planet

Quote from ThorukDuckSlayer >>

I love the idea of something which messes with metal (not least because it makes the Druid feature of no metal shine a little).

Just a room where the floor is an immensely powerful magnet - all non-magical metallic weapons and armour are pulled downwards, making it difficult terrain (1/2 movement), disadvantage on attacks, reduced range for metallic ammo and cannot jump.

Bonus points if this then turns out to be a puzzle, where the door is on the ceiling and you actually need metal in your shoes to climb up!

 
I made a cursed version of the immovable rod that did this. Basically, it would cast lightning lure on anything metal near it and create difficult terrain around it for anything made of metal, or wearing metal armor. Eventually it would build up enough weight in metal on it that it would fall down and shut off.Similarly  I once had a giant crystal that cast heat metal on everything in a room and also had a bunch of animated weapons that would attack. 

Quote from BoringBard >>

Everything is strangely clear as if the characters are looking through a film.

Then use a classic! GELATINOUS CUBE EM'!

Or one I'm using, have part of a well become a gelatinous ooze.

Or use mimics, gargoyles animated creatures, prisoners as Medusas. But nothing beats the good old ooze (theirs several different types).

 A good monster for medusa victims is stone cursed I love these guys, they're like earth elemental zombies.  they also make good living traps. A court yard of statues where if you get close to any of them you could  facing a stone claw. 

Bear trap with dynamite attached, also drow poison, and Othur fumes.