Add the following events on Halloween:

• The normal “midnight” effects should begin at 6 PM.
• Intelligent monsters may generate peaceful and with candy bars, which they will give to you when you chat to them. Or possibly, this only happens with monsters that have “homes” such as shopkeepers, priests, and peaceful Minetown inhabitants. (Not specified what happens if a monster generates with a candy bar, but you save the game and come back to talk to them once it’s no longer Halloween.)
• Other monsters have a higher than normal chance of generating with attack wands.

Tourists encounter a new shop, a gift shop, on dungeon level 1, possibly built around the entrance stairs. The gift shop stocks mostly useless items like fake Amulets of Yendor, juice and candy bars at high markups, plastic figurines of low-level monsters, and T-shirts (which are much less of a big deal to the Hawaiian-shirt-wearing tourist).

When you are carrying a candy bar, ants are attracted to you, even if peaceful. Candy bars should also be a treat for pet ants.

A bag of tricks occasionally spits out a candy bar instead of a monster (or perhaps along with / carried by a monster), but only on Halloween.

Candy bars should be colored something besides brown, since there are already so many brown comestibles. The wrapper color could be anything really, but blue or bright blue might be a good choice since there is currently no food with that color.

Very rarely, when eaten, candy bars have a Golden Ticket inside, which does something good like grant a wish. (Requires some method of not being farmable; possibly, polypiled candy bars have obroken set, and obroken candy bars never have a ticket in them.)

Add Easter eggs as an actual item, in addition to monster and chicken eggs. Possibly they have a chance of containing a candy bar, or a figurine.