Hi Amanda,
Thanks so much for visiting the Disney Parks Moms Panel with your question. October is one of my favorite times to visit Walt Disney World. The crowds tend to be a little lower because most schools are in session and there are no major holidays, and the weather is a little cooler than it is in the heat of summer, but still warm enough to be really nice.
Speaking of weather, I do think that it is reasonable to plan to visit a water park in October. The average high in Orlando during October is 85 degrees Fahrenheit, which is plenty warm, I think, to splash the day away at either
Disney's Blizzard Beach Water Park or
Disney's Typhoon Lagoon Water Park. You might want to have a flexible schedule so that if there are days during your visit that are a little cooler, you can avoid the water parks on those days.
I love
Mickey's Not-So-Scary Halloween Party! I think that you should definitely attend if it works in your schedule. One thing that I suggest is to have your whole party dress up in complimentary costumes. My family recently attended this party with my daughter as Tinker Bell, and my husband and I as Tinker Fairies. The three of us garnered a lot of fun comments as we walked through Magic Kingdom sporting our wings. Definitely take advantage of trick-or-treating - it is a lot of fun for the kids. Finally, really try to see the special entertainment that is not available at other times, such as Mickey's "Boo-to-You" Halloween Parade.
I hope that this gives you some thoughts on planning your October trip, Amanda. Please do not hesitate to ask if you have any further questions!