Personally, I'd go with a haunted house for the gym. Fake ghost pokemon props giving ppl jump scares, mixed in with real pokemon. Except all my pokemon would be fighting type pokemon dressed up like ghost type pokemon.

Ash:
"Haha oh what a jump scare! That's a really fake looking Gengar puppet."

"Oh now that one looks more like a Gengar."

Hitmonchamp uses Mega Punch. It's super effective. Ash gains concussion.