There will also families in their costumes. Our first cruise was a Halloween and our table mates were all dressed as some type of candy. I can't imagine how much luggage they brought since they were also all decked out for pirate night and formal night.
We did all three, too. We had an entire suitcase devoted to costumes and a hanging bag for more pieces of costumes plus our formal stuff. Then a whole other suitcase for regular clothes. It was a bit to haul. I'm envisioning more clothes for Alaska though. I feel like I'm packing for three climates! But that's another story...
For our family, we adore Halloween and knew, historically that sailings Sept-October will have those activities. But other than the pumpkin tree in the lobby and a few window decals here and there, you can choose to skip all the other activities. One night will be designated as Halloween and there will be people in costume, otherwise the rest of the cruise will be mostly the same. Just look for those events in the Navigator and avoid the atrium during those times. It's pretty confined to the atrium, other than the Jack and Sally thing, which will take over the Walt Disney Theatre one afternoon. (It doesn't replace an evening show, instead it's a matinee. Again, easily skipped.)
I'm sorry you don't like Halloween. It's my very favorite holiday! But then, in a former life, I was a costume maker (for theatre), so that time of year, I get to channel that part of my life again and it fils me with joy.
I think that's a great thing about Disney - there is something for everyone. I'm sure you'll still enjoy your cruise!
You might want to look at past Navigators on disneycruiselineblog.com for ideas as to the activities there are to help you make your choice. That's very generous of
DCL to give you that option.