Cool, we'll be there the Halloween night, too.
Offhand I can't really think of other stuff going on besides decorations of course, and the seasonal haunted hayrides over at Ft. Wilderness.* Atm I'm not really remembering anything else that stands out as an official Halloween Activity. As for what park you spend Halloween Day in...that's really just a matter of preference. It is a Saturday this year, plus it being Halloween, so I would expect things to be pretty busy all around. Of course, the MK might be worse than others, as it often is, but that may not turn out to be true.
* They may not be doing the hayrides this year, at least not at Ft. Wilderness. I suggest calling for information if you're even interested in that. It sounds like it may be part of MNSSHP now, but not included with tickets.
We have talked about maybe spending a lot of the day in the MK as well. In theory I liked the idea at first, but I'm thinking that in order to leave, grab something to eat and properly put on costumes, we'd have to leave the park kinda early anyway to get back on time. Plus there's the added bonus of being sweaty, disheveled & tired before it's anywhere near time for the party. In my house, I'm kinda the one who actually thinks of these things, before they cause all sorts of problems. lol Since you can generally get in a lot earlier than the time on the tickets anyway, I'm leaning much more towards relaxing at the hotel, swimming etc. Having lunch there, then doing the whole shower, costume thing and moseying to the MK. Since we don't have APs right now, that way we also won't end up wasting a pass for what would truly end up being such a short day before the other ticket kicks in.
If I was on vacation and had limited time to visit the parks, then I suppose that would change my thinking.