If you're looking for a simple, fun toy to amuse the kids with and you don't mind trying a little ventriloquism, I still recommend Crazy Metal Head and Crazy Skeleton from ezone. From the same company there's Crazy Pumpkin, a simple but fun facemaker that little kids will enjoy.
You may also want to try a magic trick or two, as we've covered a few. My personal favorite is from the Tenyo company, called Magic Shuffle, it's like having a deck of cards in your hands and performing a simple but baffling trick.
Some other ideas: get a flashlight app (pretty much all of them do the same thing: light up your iPhone 4's LED), how about a hypnotic swirling app? Or grab a drawing app to keep little monsters occupied as you drive to your local "trunk or treat." Also, if you're dressing up as a Starfleet officer, my choice for a Tricorder app is Tricorder TR-580, and here's a tip: the old Apple "holster" case for the 4th-gen 20GB iPod fits my iPhone 4 almost perfectly. More importantly, it looks a lot like the holsters Starfleet officers kept their phasers in!
eBooks on the iPad really shine, as David Winograd pointed out with a couple of titles the other day. Dave Caolo said he's enjoying some from PadWorx, and there are a few titles in iBooks to keep you keep you entertained as well -- except Harry Potter, as J.K. Rowling will not allow digital versions of her books.
There are some HD magic tricks available, and as an amateur magician I can recommend Amazing Magic Match HD for its ease-of-use and quality graphics. If you're whipping out a box of matches to light a jack-o-lantern, you can likely do this trick with a little practice. Others that are fun: Marty's Magic Coin, Magic Card or learn a bunch of tricks with Learn Magic! and Mental Magic! There are really a lot of ways to do magic on the iPad using notepad apps and calculators, but I'll leave that as an exercise for the curious reader to investigate on his or her own.
Lastly, If you're looking for a fun diversion that is somewhat of a knockoff of Pocket God, check out Pocket Devil HD. It's more gruesome than Pocket God (yes, that is possible) and it is suitably themed for the holiday. Pocket Devil has some mini-games (as Pocket God does), and there's an iPhone version as well. It is rated 9+ and I would caution parents of younger children -- while Pocket God may be cartoony-violent, Pocket Devil inches up the gruesome factor a bit more and is definitely not for younger kids.
We've only scratched the surface of what's out there, although the vast majority of "Halloween" apps are gimmicky and will not stand the test of time. If there's something you use beyond the holiday, leave your suggestion in the comments.