Magic mouth can be a fun one to play with.
Cast on all your individual items to raise an alarm if someone tries to steal it. You're not going to make your spot vs the rogue's pickpocket, and now you don't have to
Put 'em on stones, so that when they're out of the bag if anyone comes near w/o speaking the password, they raise an alarm. Set the rocks out at night and sleep peacefully.
Put 'em on arrows to say WHHHHEEEEEEEEEEE when shot.
Put 'em on arrows to say something after being shot, to distract guards.
Put 'em on your clothes to say "good job" when you cast a spell
Chain 'em so that this can go on all day (by which I mean the first MM goes off after the first spell, the 2nd goes off only after a spell and if the 1st MM has spoken, the 3rd goes off after the 2nd, etc)
Put 'em on the fighter's shield to say "Not in Face" when combat starts.
Put 'em on a sword to say "Oh, the humanity!" when it gets plunged into someone.