DLR is a lot of fun in October because HalloweenTime should be in full swing. Assuming 2016 is similar to past years, Haunted Mansion and Space Mtn should both have their Halloween overlays in place (Haunted Mansion Holiday and Space Mtn Ghost Galaxy, respectively). There should be seasonal foods (some that are Halloween-themed, others that are Fall-themed). Some of the park will have Halloween decor, but it's usually concentrated on Main Street and in Frontierland, but with the closures for Star Wars Land prep, it's not known how much that will impact HalloweenTime this upcoming year. Some of things that had been historically presented during HalloweenTime were not presented this year due to the 60th Anniversary Celebration. With the Celebration official ending (at this point) on Labor Day (Sep 5), it's not clear what they'll do in 2016. For example, Mad T Party (in DCA) usually has a Halloween T show they present during HalloweenTime, but in 2015, they continued to present the Diamond T Party (so no Halloween T). I'm hopefully that Halloween T will return or be mixed in with Diamond T, but we'll have to wait and see.
As mentioned the Big Thunder Ranch area is being closed permanently in a few days (Jan 10 is the last day), so there will not be a Halloween Carnival in 2016 (they really wasn't one in 2015 either). It's not known if any of the Carnival elements will be moved elsewhere or not (I'm kind of inclined to say it won't because they didn't do anything in 2015).
On select nights in October, DL presents Mickey's Halloween Party (MHP). It's a special event, so you'd need to purchase a separate ticket for it, but it's a lot of fun. They have historically presented a special Halloween fireworks show (Halloween Screams - Jack Skellington is the host and Zero flies!), there is trick-or-treating for guests of all ages, special entertainment (e.g., the Cadaver Dans), and unique character meet and greets (e.g., villains). In the past, they have presented Mickey's Halloween Cavalcade (a medium-sized parade), but this year they presented Paint the Night instead, so no one knows what they'll do in 2016 (I'm hoping for a return of the Cavalcade). Last year, they announced dates and began ticket sales in July and all but one of the parties sold out in advance (you can see when the parties sold out in the
2015 Calendar of Events thread). For a detailed review of MHP, check out the links in my Signature.
It's worth noting that at this point we don't really know much about what will be happening in and around DLR in October that might impact crowds, but the UEA Conference is slated for October 20-21, 2016, which tends to mean high crowds in the parks because Utah schools are not in session. There is also the potential that IaSW may be closed for part of your trip. IaSW usually closes in mid-to-late October to have the holiday overlay installed (last year it closed on October 19, 2015).
