Some considerations:
 
Spiders are scary. Having bait in a spider egg sac - snap! - was considered as a possibility.
You want the spider to look, move, sound, and maybe even feel realistic. The sound of a servomotor tells people this is made in China, not a realish spider.
Invisible string is doable. You can get it from Michael's, or different strengths from the fishing store.
Twisted elastic bands, unlike bungee cords or normal elastics, have a steady pull from start to finish.
Consider lighting.
 
 
 
 

Big hairy spider. Three alternatives are being considered, all triggered by a kid grabbing a chocolate bar. No idea how to engineer the trigger, with a light move leading to powerful effects.
 

1: Have the spider rush across the grass. Sound nice, but having the spider move straight rather than chaotically flopping involves some challenging engineering. Also it's difficult, at least for me, to attach string to the ends of the spider. This uses part of a coat hanger hammered in the ground, and a keychain ring for the string to slide through.

There might be a crazy solution to the straight issue. String attached to the front and the back of the spider, and the string is in a fairly tight loop across some grass.
 

2: Have the spider at the most two feet away from the trigger. The spider quickly but quietly moves forward and down (and maaybe goes back up some). Simpler engineering, we don't have the issue of the spider moving straight, tied in the middle of the spider. This is what a spider does, and the movement of the legs is more natural. Genuine and scary experience, in theory. But in practise? Working on this one. A bungee cord yanks back, with a (likely invisible) string that goes through a loop yanks forward and down.
 

3: Spider jumps from the garden to the bottom of the railing on the steps next to the front door.

Pro: This would make a great experience and a story to pass on. The spider looks both visible and creepy by the bush in the garden, staring up at an angle. The spider would pop up - think of the facehugger in the movies - and lands at the bottom of the railiing, staring up, not too far from the trigger. Because it's at the bottom of the rail, you would not see the more artificial bottom of the spider. The railing might make the trigger engineering easier.

Con: The power side of things would be a headache. Where exactly would it go? Things can go wrong when the spider goes to the bottom of the railing. For example, it could be flipped over.
 
 
 
 

Some lesssons recently learned: 1: Keep the attachment (between the string and the front of the spider) raised, not headed to the ground. 2: Keep the spider straight somehow.

Moving toward twisted elatistics rather than bungee cord. Mostly because of the nature of the movement, but there would also be less stress on the invisible string. But how would I attach a twisted elastic to a string?

Is there an alternative to the chocolate bar trigger, such as a tripwire?
 
 
 
 

Should we try the egg sac? Scary, but too many problems.

Having a carrot stuck in the ground as a trigger would be odd, but mechanically good. Do you want to feed the spider a carrot? By the way, that's another reason to use twisted elastics, less pressure on the trigger.

Pulling a raised spider from the front will make the spider flop around. Right? Testing shows that the straight problem has probably resolved itself.
 
 
 
 

I'm thinking of making a "Beware of spider" sign for the front. But that depends on the spider being successfully made, and that is a failure every year.
 
 
 
 

I gave this a try, indoor where it is warm and visible. Working on a novel contraption with tension and, above all, invisible string, is discouraging. Thin string breaks, while strong string is visible. I told my dad it could be underwhelming, but it was worse than feared. The spider jerked forward a minimal distance, then tumbled on its face. My dad argued that the legs provided friction, so the string and spider should be more raised up. I argued that the twisted elastic bands - in a loop by the way - should be much longer. Giving up is an option.

Fishing around for other options. What about the spider simply pushed down suddenly as a rattled kid is trying to get the chocolate bar under its fangs? From an engineering point of view this would be both easier to do and should work. Now the focus goes to how exactly the trigger works. Would this actually be scary, or at least remarkable? Silent? Yes. With kids' noses close in there, things will have to be more hidden, maybe some spiderweb obfuscation.
 
 
 
 

It's stuck on the options for the trigger. Also time is running out. Will the annual failed attempt to make a Halloween prank/trap fail?

This idea? No. That plan? No. The other scheme? Won't work.

There has been some progress. Get a mousetrap, strip out the spring and the yellow bait thing, so you just have the flapping bar for leverage. This works. Although there is a problem with the string and bar hinge. Camouflage is a whole thing. Attach to the ground somehow. Muffle the mousetrap sound. The nice thing about this is bait and string don't touch each other. I'm thinking of using a banana. Odd, but it has weight, also visible.

What about abandoning the mousetrap and creating something from scratch that is functionally the same? I think I'll do a mix of the two. A U of coat hanger jammed in the ground, the flopping mousetrap bar by itself, linked by a bit of strong invisible string, or maybe one of those hair things. The mousetrap sound problem is solved, and the camouflage problem is solved. Held down by a banana. The scenario is

The spider is more in the shadow on the lawn, showing some leg in the light. I wouldn't go to that block, never mind the house. At least it's not alive.