Rotten eggs stink more than you could ever believe........
A chemical called "red indicator" if placed with red wine (communion wine) will cause all who drink it to pee red as it went in and scare the shit outta them. If you wish , you could add ground up riboflavin tablets which if taken in enough quantity will cause extreme hot flushes and yellowing of skin.
A glass jar with a tight lid, filled with draino and a dead rat and hidden somewhere nearby inside is also a good one. its festers until the body gases causes the jar to explode , spewing its contents everywhere, and its a stench that wont just be scrubbed away.
Squirting parishioners cars with brake fluid is an oldy but a goody, as is hiding dead crab meat in their heater vents.
the list goes on really.