Ahh, the perfect thread for me!!
I live in an apartment, so no Trick or Treaters, but my roommate and I love Halloween so we've decorated like crazy.
We keep it very frugal, though. I decorate for holidays with the four Hs -
1. Hand-me-downs (for example, this year I got a lovely Day of the Dead sculpture from my Abuelita!)
2. Handmade (crochet and construction paper)
3. Harvest (mini pumpkins, apples, and candy corn as centerpieces, all of which will be eaten after the holiday is done)
4. High-quality (I ask around the overpriced home goods stores to learn when the holiday decorations will be 50% off, then show up on the first day of that sale and buy up to $25 worth of decorations each year, looking for high-quality make and reusability. I'm slowly building up a collection of classy Halloween decorations, including embroidered pumpkin napkins, spider napkin rings, a spider web cake tray, etc!)
For the construction paper.... it's so easy to cut out bats and ghosts to put on the walls. I host a big Halloween decorating party with my friends the last weekend of September. It's fun, social, cuts down on decorating time, and leads to some seriously creative results!!! For example, I got busy cutting out bats, then glanced over at the fireplace and discovered that it had an eye, teeth, and even a lolling red tongue! (My sister and my childhood friend were giggling and snorting, trying to look innocent.)
A specific craft I've been doing a lot this year is soaking the labels off of jars (apple butter and moonshine, both mysteriously emptied), filling the jars with candy corn, and then putting a little candle inside to flicker and look pretty. Easy, festive, and frugal!