Okay, I may be late to the party here but I have a campaign that features Hell prominently. Some places I have:
(I'll start by saying that Hell is supposed to be horrible and inhospitable, so not everywhere is PC-friendly)
-An endless sea of diseased blood. There are piles of humanoid bones so high that they peak out from the blood here and there. There is a red musk in the air that blocks sight beyond 60 feet and forces Fortitude Saves to avoid becoming Nauseated every few minutes. If you becom nauseated, you can't stop until you succeed on a Fortitude sSave or breathe fresh air. If you become nauseated, you must also make another Fort save or become diseased, since the musk is just the blood particles floating about. Make sure they know that they are breathing human blood, and that they can hear the screams of those being bled somewhere in the distance.
-Miles of fissures and rock... with fire instead of air. Doesn't matter how it works, everything is burning, even the air you breathe. The closer to the bottom of the fissures you get, the hotter the fire (2d6 fire damage rather than 1 d6 per round).
-Endless murky magical darkness. Any light only illuminates within 5 feet. In that light, you can see that the floor is squirming--it is in fact, an entire plane of living bodies piled on top of each other, bound together by barbed wire. They can smell sweat and blood mixed together, and can hear moans of pain whenever they walk through the darkness. Obviously, walking on such unstable footing equates to about 5 foot move speed. The barbed wire itself may actually attack them, using an improved grab/constrict combo that does hp and WIS damage (making them go crazy), and when they have lost their will, they are integrated with the floor. (By the way, there is no ground beneath the bodies. Digging will only get you dirty)