Here are a few good ones that I remember
Wagon drives under a giant spiderweb over head which drops about six or seven feet on top of the wagon, It does not touch them but it comes close. The whole wagon normally ducks and screams. simple rope and pulley to operate.
The wagon drives by a billboard for the haunt and an actor with a chainsaw cuts the post and it looks like its going to fall on the wagon but it stops short. Great reaction from the crowd.
Another one is a basic spark wand but the chainsaw actor connects one lead to the wagon rail and the other is on his saw blade, so the saw shoots sparks as it hits the rail. pretty easy to build and a great effect, as the wagon drives off the (jumper cable style) gripper pulls off by its self.
Im pretty big on stilt walkers and they are great for attacking hay wagons, especially the fast moving jumping stilts.