Mapping the Planes

In my upcoming campaign there will likely be a number of expeditions to the Outer Planes. It's not exactly a Planescape campaign (it's set in Greyhawk), but I figured you guys would most be able to help me out with this. My campaign will be a "sandbox"-type campaign, so the players will determine when they travel to the planes and where. Fortunately I have an idea of where they'll likely head, including two Abyssal Layers, the Demonweb and Shedaklah.

So the challenge is, determining what they will find when they go there. Mapping would be ideal... except it can sometimes be difficult to map the Outer Planes. Really distances aren't as meaningful, and the terrain is sometimes difficult to map, (i.e. Mechanus). Nonetheless, to run this campaign I must have the information in some form. I have gotten some good advice elsewhere, but I wonder what you guys have to say.