Jace: Send an army (or more if you want to!) of ducks to him, dress his daughters/sons as ducks, decorate his room with ducks, throw a party with ducks as the theme, anything related to ducks

Magnus: Steal all his makeup and glitter, get rid of all the glitter, 'kidnap' Alec, give him makeup/glitter that dyes his face in a certain color you want

Alec: 'kidnap' Magnus, have Clary or any other woman flirt with him, put Magnus in a dress without glitter, have Magnus cover his bow/arrows in glitter that sticks to your skin when you try to pick it up

Clary: have Alec flirt with her and be nice for a day and say sike at the end of the day, 'borrow' her art supplies and her stele so she cant draw anything or runes

Izzy: 'borrow' her whip, have Simon/Alec pretend to propose/flirt with Clary, replace her demon-detecting pendant/necklace with a fake one

Valentine/Sebastian (the fake one): give them a fake cup, (for sebastian) give him a fake Clary and record him