Cheat unit recommendations:
Cowboys: Watch the computer reel in horror as their musket formations are mown down by squealing, drawl-ridden six-shooters! (PS - I am from Texas)
Submarine: Historically, there were submarines (or a submarine if I remember correctly) used in the American War for Independence. I'm talking a one man job controlled by a pedal crank, but sit and munch with glee as an anachronistic 16 inch Ericsson with a 300lb warhead leaves a magnificently pixellated trail as it heads toward your enemy's unsuspecting frigate! "What is that thing? A fast fish?"
Neoclassicists: Weep in your Cheetos as Joseph Addison converts your industrial military barracks into the gladatorial pit of yore! Who needs saltpeter when we've got the bronze dagger? Caesar didn't need guns! Why he conquered most of the known world with weapons that teenagers can buy at Scarborough Faire! And so will the Neoclassicists conquer you by reducing your advancements to nothing more than mere plumbing and hide jerkins.
Dr. Emmett Brown: "It'll shoot the fleas off a dog's back at 500 yards... and it's pointed straight at your head!"
The cast of Little Women: Lose all taste for your yummy deoderant stick as these prudish Victorians teach your well-trained army how to hide their feelings and speak in sentences as long as Bible Chapters! Their presence equals greatly removed unit speed and an almost sympathetic yearning for Henry James.
AT-ATs: No snowspeeders? Big problem. The galaxy's a big place, but you can't have an Age of Empires without THE Empire! Dun dun dun da da dun da da dun...
Nate Griffin: Poisons all your settlers. We won't go into how.
Finally...
The old dancing Six Flags guy: Wipe away those tears as your new cheat unit races through the enemy bases in a shamelessly overdone bus! He steals all their units away for a weekend at Six Flags while leaving your units to continue their humdrum life... of destroying the enemy's unprotected base.