DiznEeyore, I can't remember all of the details from when we were there last year, but I think they had several activities during the day on Halloween day (we didn't participate in the day activities last year, just heard about them). The one thing I can remember is that they have a pumpkin carving contest -- I think they even provide the pumpkins! Then they have the judging of the pumpkins (and some very nice prizes) at the campfire that night. You should have seen some of the amazing pumpkins people did!
As you may know, FW has a campfire program every night, including a "sing along with Chip and Dale" and a Disney movie. On October 31, Chip and Dale are there in their Halloween costumes. Toward the end of the sing along, the headless horseman rides through the campfire area (very cool) and drops off a big black bag. Chip and Dale are "afraid" to look inside, but when they finally do, there's candy in there! So they hand it out to all of the kids -- most of whom are in costume. Then they show two "scary" movies -- last year it was The Legend of Sleepy Hollow and The Black Cauldron. As with every night, there are campfires set up to toast marshmallows, and you can buy the stuff to make s'mores as well as other snacks at the "Chuckwagon." (You can also bring your own s'mores stuff.) It's a lot of fun! I highly recommend doing the FW campfire at least one night, even if it's not Halloween night.
There's also a Haunted Hayride, which I've heard is fun. We didn't do it last year, but plan to this year. This costs extra, but I think it's fairly reasonable.
That's all I can remember -- sorry the details are so sketchy! (Maybe someone else can fill in more info?)