Here's a simple Halloween idea I'm going to use for the first time this year. As demonstrated on my office door here above I have a floating ghost projected on the frosted glass from the inside.
All I did was take an ordinary digital projector, hook it to my laptop and loop a YouTube video of some floating ghosts and point it the inside of my office door.
Now I'll take this home and over the weekend find something like a cheep frosty plastic shower curtain, or a white bed sheet to cover some windows on my front porch to project on. You don't want it clear, nor do you want it completely opaque. You can purchase window cling film explicitly for this that will give you the best resolution. I'll connect some speakers for creepy sound and I'm nearly done with my whole decorating plan.
Speaking of best resolution - If you have a HD projector there are several places online where you can purchase quality videos you can download along with the window film and yard screens. Otherwise places like your box box home store often have cheap projectors for around $60.
It looks better when I turn off the office lights. I can't wait to see what it looks like when I set it up on the front porch over the weekend.