On a serious note, I know that not one Jew has EVER participated in trick or treating or attended anything to do with this holiday.
Women: Halloween is a time bound mitzvah, but women are nevertheless chayav to keep this precious mitzvah because without girls dressing inappropriately, there would be no partying, and without parties the next mitzah would be tough to keep.
B’Simcha: Everybody is chayav to be b’simcha on this chag. Now, it’s unclear whether or not a seuda should be held (the poskim argue), but getting together with friends, dressing up, drinking, and touching the opposite sex is mandatory. (Anything assur on Yom Kippur is mandatory to do on this day).
Candy: Most Modern Orthodox Jews hold that you need to eat at least a kezayis. However, the Jews who keep Shabbos but eat non-kosher dairy and parve food require that your candy intake be equivalent to your matzah consumption on the first seder night.
Prayer: The sacrifices on Halloween were similar to the korbanos in the beis hamikdash, therefore there are special prayers to be said on Halloween. Most poskim hold that singing along with the songs at your party constitute prayer.
Costumes: If chas v’chalila you missed Purim for whatever reason, the rabbis have concluded that you can still get schar by dressing up on Halloween. Consider it Purim Sheini.
Trick or treating: Persumei nissa is a highlight of this yom tov. In order to do that, however, one must go from house to house collecting candy (b’chatchila). The halacha says that one must comply with all of the laws of Seder Nezikin while planning the trick (b’dieved). Because goyim aren’t people, you can do whatever you have to do to fulfill the mitzah of trick. (Poskim argue whether or not trick or treat is one mitzvah or 2 different ones. One man d’amar holds that “trick or treat b’dibur echad” so it’s like one. All poskim are maskim it’s mehudar to do treat).
Pumpkins: As an extension to persumei nissa, one must display pumpkins outside his/her house during the night of Halloween. All the halachos of menorah apply, but keep in mind that women should be working during this time because they need to support their husbands in kollel.
Shacahris the next day: Because you’re supposed to be hung-over and it’s an abomination to daven while sick, we are patur from davening shachris on November 1 . Tefilin should be worn at mincha.