The M1 can be a lot of fun for Halloween! I've only scratched the surface - I have an input and output expander in my garage in this house that would be extremely easy to run wire from there to the front door to trigger things, but no time this year.
What I have done so far:
I use an Elk124 Recordable Voice Module in-line with my existing keypad speakers, of which there's one right by the front door. Whenever someone presses my doorbell, there's a glass shattering and witch howling sound. I also have two other creepy halloween sounds and alternate between which one plays every time I open the front door (creaking door and spooky sounds).
You can also do motion triggers on your props (most have a demo-mode button that can be hacked); I had one prop that wouldn't trigger well on its own so I hooked it into the doorbell trigger - worked like a charm. Almost any prop can be hacked because of that demo-mode - then it's a matter of when to trigger it. You can also get those pressure pads really cheap and use them as inputs so as people walk up the path they can trigger things - they're ugly though so they need to be put under a better looking mat. Otherwise motion sensors work well too... Having a WGL W800RF32 integrated with my system, I can use those super-cheap outdoor X10 motion sensors to trigger anything I want as well.
I'm sure there's a lot more - but that's my starting point.