dog vs ninja

All that you need for this is a stack of index cards, each with the name of possible combatant written on it. For example: samurai, tiger, battleship, pro-wrestler, ninja. Make at least 30 or 40 cards.
Shuffle the cards and choose two, then ask your students who would win a fight between the two combatants named. The idea here is to get your students talking and, if possible, speculating. Ask them why they think this or that fighter would win. Is he / she stronger, larger, or faster, than the vanquished foe?
As always, its good to add a few comedy entries in the list of fighters. How would "the smallest dog in Osaka" fare against a bull? How about Tom Cruise vs. a chicken? Godzilla vs. Madonna? Ichiro vs. an army tank? You can also add the names of students - this always gets a laugh, and can cause animated debates among your kids.
The great thing about this format is that it can always be randomized, simply by re-shuffling the deck.