Good idea.
Garbage can
This one's for a dog. Fill a garbage can with sticky things, like stickers, velcro, and pre-sucked candy. Then put a treat at the bottom and wait for your dog to go all crazy on the garbage can, getting the sticky trash all over himself. Bonus if he'll wear the can on his head for a while. Also works as an Oscar the Grouch costume.
Mud monster
Just let your dog roll in mud. One problem with this one is that no one will want him at your Halloween party.