[Biggie]

You could do something like the jeepers creepers monster, that has always come to mind on a haunted hayride. Werewolves, they would fit in if there is woods. Then lastly, maybe some pissed of lumberjacks. The chainsaw is a mighty tool.

 

[Biggie]

Have a victim on the train, as an one actor who gets kidnapped. They can also through out the ride can add to mood. They can point into darkness and scream, then from behind comes another actor and jumps on the trailer. They could tell stories to people, like "I can't believe they did a hayride through here, this place is damned" And just add little comments "what was that", "Did your hear something", "OMG I saw something".

On that stretch of road have acouple of the actors or volunteers put there cars on the side with doors open and hazard lights on. Next to the cars dummy bodies, with little touches to the body to make people curious of what happened. Like scratch marks down the chests, missing limbs.

I've always wanted a random giant birds nest that is human sized with a huge bird that flys overhead.

 

[Biggie]

Well if the witch uses animals then I suggest a goat man, they look freaky and imply satanic happenings. Its a local legend from the county north of me, there is a really amazing haunted house called goatmans hollow.

and you can make the actor digitigrade stilts, did a pair for last year. Great effect in my opinion, height and goat like. Thats not me btw
Asterpix Interactive Video - Arbiter Digitigrade Stilts test, June 2008.