An old standby is to have an agent of the church steal an item from the party. It doesn't even have to be something particularly valuable, or useful, though obviously that helps.
Originally Posted by Deophaun
It doesn't so much as demean the celestial monkey's existence, so much as fulfill it. Without the ability to be summoned to set off traps, retrieve objects from dangerous situations, and all and all be a party's guinea pig, the Celestial Monkey would languish in obscurity in the MM and do nothing more legendary than eat celestial bananas.