Depending on when in October you are going, you might see my favorite "celebration" at WDW. Beginning in mid October and lasting through mid November (this year the dates are October 18-November 16), EPCOT hosts the International Food and Wine Festival. There are new kiosks/booths up around the World Showcase, a (small) winery tour, and some new flowers and plants around the topiaries (for example, gardens of different herbs or kinds of peppers). Whether you are an expert on food and wine or someone who just wants to try something new, you will find all kinds of interesting (free!) experiences there. They host culinary demonstrations and wine seminars nearly all day every day of the festival.
They don't put up fall decorations, like they do at Christmas. You won't find much as far as colored leaves (except what happens naturally) or the smells of pumpkin pie and mulled wine -- unless you find those foods in a restaurant. But the smells around World Showcase are wonderful -- yummy foods at every turn!
To me, that feels like fall because we always spend our fall vacations there.
Have a wonderful time and enjoy the quieter days and (usually)pleasant weather of October at WDW!