I love Halloween especially as its the beginning of the Celtic year for me. Can't say I've ever gone in for making decorations for it but after some thought came up with these as ideas for you if you want them.

Cauldron: Use a large mixing bowl or the biggest ballon you can find depending on the size required to build on. I have started a very large cauldron myself but its not for Halloween. It takes a 'LOT' of layers to get it really strong and you will need to use something very sturdy for feet because the whole thing starts to get heavy if you choose to do something as large as mine. I'm now in the process of decorating it, not staining it yet but adding relief work for raised detail. I should imagine a cauldon would be good to put candy in, or make slightly smaller ones with handles for kids trick-or-treating to collect their bootie in.

Black cats: Empty bottles work as a really good shape to build on for this.

Smaller monster, witches and skull heads: Just from balls of news paper, cover with pm. String them together and hang them like garlands when finished.

Spiders: I image a ball of news paper and some pipe cleans might work to build on. String them up when finished.

Ghosts: Ballons might be handy to build on, then either cover with white cloth and draw a face on or allow the balloon to deflate a little before the pm is total dry and crinkle it a little around the largest end. Try and keep the narrow end rounded for head. When its completely dry trim the larger end to make it look like torn cloth.

You could always make some witches and ghoul figures but it depends on time I guess. Anyway goodluck.

I'm probably revealling a warped mind but had you thought of making a blasted tree?  I'm thinking of something big enough for a person to hide behind/inside.  Possibly with short branch sporting a natty noose and tattered bits of decayed cloth.  If the tree is built with the back open a person can be hidden within and when the little tykes come calling a hand can emerge from a knot hole with suitable treats.
It's a large project and would be a pain to store afterwards but I do think it would be fun.