Even though temperatures in my part of the country have not yet wavered an iota from all-time high summertime highs, my thoughts turn to late October's festivities. Though we only get about four days of what any normal human would call "fall", I have always enjoyed the season.
Steampunk should not be marginalized to one day per year, but after all it is the one day of the year when almost anything goes, clothing-wise, which equals creative freedom! Combining steampunk and Halloween may not be such a reach after all. The ever-popular mad science has a place in the genre, for sure, and who can resist drinking their mystery punch from lab equipment? Not me! (Watch out for that Pyrex glass, though. If it hits a tile floor, you will see a demonstration of total disintegration!) It is also a time to share steampunk with people who may otherwise not know about it -- co-workers, for example.
We all know what steampunk is to us. To adapt it your decor to the season, think grandmother's attic. The darker side of Victorian decor can create a spooky aura. Brains in jars suit the bar and food area, but more traditional pieces can help the rest of your home. Try eerie black-and-white photographs in ornate frames for the walls; I have heard tell that there are wall decorations that change images as you walk past so that you feel watched. Faux cobwebs and ripped gauze add age to the place for pennies (unless your house is like mine and occasionally has real cobwebs already, then you're all set). If your furniture is less than Victorian, you might cover chairs and tables with vintage-looking blankets or cloth.
As for your costume, just delve into your favorite idea and dress the part from there. The mad scientist, corseted pale Victorian lady (or go more gruesome at will), experiment gone awry, airship pirate (or captain), greasy dieselpunk wonder mechanic are a few options to consider. If you want some thoughts on how to put together a basic steampunk outfit, read this post I made on the subject, or if you want to make your own goggles, here is my take on them. Most people's internet posts regarding their own steampunk costumes involve some variation on the statement, "my Halloween costume is finished! I am a steampunk... something. I think." But it is the fun in putting an outfit together that is important, so no worries! (Last year, I got asked if I was "Hellboy", because of my goggles, but I am over that. Mostly. lol)
Though I have no association with this site, Grandin Road, I spent over an hour perusing just their elaborate selection of Halloween selections and found them pretty interesting. They have indoor decorations, outdoor decorations, costumes, you name it. They even have the Martha Stewart line of Halloween grotesqueness.... (Yea. I thought it was not right, too, but there are some decent pieces in there.) You can also find a link to other decorating sites in the "Tag Cloud" on the right of this page, under "Halloween."
And last but not least, pumpkins! Or the carving thereof! From UniqueScoop.com, I give you 100+ Creative Pumpkin Carving Ideas. I would have named it "many strange things to do with pumpkins," but I digress. Here are more pictures of carved pumpkins on Waylou.com, as well. I always feel a little bad about wasting a perfectly good gourd, but then I remind myself that at least the cheerful, large, orange, seedy fruit will not go extinct as long as the holiday is celebrated, so it will be okay. I like the collection of original "geeky Jack-o-Lanterns that do something strange and unexpected" displayed in this article on MentalFloss. The Instructables site has dozens of pumpkin videos. Carve that thing out, add LED's, goggles, gears, fog, motors, or whatever you have in mind to make it your masterpiece. Spooking the neighbors grants a bonus.
I hope this has been at least somewhat helpful in getting your fall off to a smashing good start. Please feel free to post links to pictures or comments about what you are working on for your Halloween outfit or gala, as I love to see the creative ideas people have in their devious steampunk minds. =)