For the past couple years, we've been in WDW for Halloween. We always do the Halloween party on Halloween night. It always seems to be busy in the walkways getting to rides, but the rides had very short lines. It's like all the people were standing around? We usually watch the 2nd parade, and really after the fireworks, the park clears out. We rode Splash Mountain about 5 times in a row one year without ever getting out of our boat. There was just no line when we came to get off.
There are activities in the resorts. Pool parties, trick or treating, etc. I also have heard that they do things at DTD, but we've never been, and that may get crowded as well, since it's a free place for locals to trick or treat.
Hope this helps. Just wanted to give you an idea of the Halloween party on Halloween night. If we were going this year in Oct, we wouldn't hesitate to attend on Halloween night again. (On a side note, the 1st year in 2 or so that we were actually home for Halloween, my kids were so bummed at the end of the night. They decided it was terribly boring being home for Halloween!)