First, a visual aid....
...where, to put it in scale, each of the squares in the grate is about 2 1/2 feet across. I happen to have this printed out as a tile, along with a couple with just plain grating, if you want to borrow them.....
Agin's Inn has a lot of these things, by the way, although most of them aren't nearly so...aberrant.
Another thought? A wall of crystal. As the PCs approach, they can see their own relfection...and as they get even closer, they can see those reflections are twisted, and just...
wrong. Weird growths, tentacles, eyes in odd places.
Except for the alienist, who looks entirely normal.
One more, shamelessly stolen from Clive Barker's
Imagica. A rivulet of water, flowing from an unseen source, that's running
uphillp. Actually flowing up a stiarcase, running into the distant darkness.
As to treasure..... An IOUN stone that gives alertness, and darkvision. And is actually a living eye, floating silently around the user's head.
A glove, made of some strange skin, that has a mouth set into the center of its palm. When worn, it provides a nasty meele attack, and allows the wearer to drain hit points from those it strikes three times a day. But it can't be removed, and at night the hand...speaks. It mumbles things, too quietly and too indistinctly to properly make them out, that are terribly, terribly disturbing. And god help you if you go more than a day or two witout letting it feed.
And finally, a sword. From the hilt, it looks normal enough. Old, and worn...well used, to say the least. But the blade! It twists, and warps. It doesn't actually move, per se, but it's not entirely set within this reality. It hurts to look at the damn thing for any length of time, but the longer you look the harder it is to look away. Could find it in the hands of some poor bastard who starved to death as he stared at the blade, sitting forgotten in some dark corner....
It's a +3 Longsword that ignores armor, but every turn that its used the wielder needs to make an increasingly difficult Will save to avoid being fascinated by it. If he fails the save he'll just stand there gazing at the blade, until someone helps him.