Another logistics thing: how much area do you need to cover with the fog? You'll need many fog machines if you want to cover a long path (and somewhere to plug them in, if they're not battery operated!), or a heck of a lot of dry ice.
Definitely think that having someone jump into the back of the hayride when no one is looking is great. Maybe having something falling onto the cart would be good too? (Maybe not confetti, but tissue ghosts
or the like?
You could also have the path take a couple of forks, where the riders have to call out which direction to go in. That way, it'd be a different ride every time!