My favourite one was always to get a spy or two inside an enemy city, deep inside enemy lines, and then beseige it with a halfstack of cavalry, with 1 unit of horsearchers or similar.
The spies open the gates, you rush the town, get to the central square asap, then fend off the enemy for three minutes. Unless you're fighting Greeks (aargh phalanxes! :p), this isn't too hard.
Then, you exterminate the town, destroy every building you can, leave with your entire army (leave no garrison), an amuse ourself building up an army of peasants which you can either leave to pretend to defend the city, or just move them into another your real cities to beef up the population there.
I love this move, especially as Carthage in Vanilla, or as a crusading force in Chivalry.