This window display was the cost of a blue lightbulb.
Having a garage full of cardboard boxes awaiting a use, I cut one out into the shape of a tree with a cat on the limb and painted it black to be a silhouette. I taped it up to the front picture window of the house.
I backlit it with a blue bulb on a desk lamp, but then you could see some of the room, so I took a ghost head I had that was a mannequin head with draped cheesecloth and hung it over a floor lamp to create a ghost behind the tree.
The effect was stunning and I realized I could do so many types of window displays like this. I always wanted to do the ghosts from The Fog with glowing red eyes and a fog machine going....
For something more lasting, you might want to get some foam board from the craft store and carve these out and store them safely for the next year.
Here's some cool window displays for inspiration -
mannequins and dolls in the window like this work great. One year, I had all the dolls and mannequin under the curtain like this with a lightning flash strobe and the kids got so intrigued looking at the display, they forgot to ring the bell for candy.
There are loads of creative ways to do your windows. Consider thunder sounds and lightning flashes
Consider a table with a science experiment of beakers and colored water and a plasma ball.
I think it would be cool to block out a window out front with cardboard and cut a hole in it (low for kids) and decorate it like a peep hole. They can peer inside to see a display.
Of course, an AtmosFX projector and cloth in a window with a holographic projection is another fine idea!
The cool thing about these window displays is that no one can steal your stuff.