The mimic spits out the promise of a brighter future.

I feed the mimic what's left of my sanity.

The mimic spits out Bel-Shamharoth. 

I feed the mimic . . . fresh calamari. 

The mimic spits out a statue of Cthulthu.

I feed the mimic Houdini's Straitjacket.

Je spits up houdinis undead body.

I feed the mimic a five star meal.

The mimic spits out the bill. 

I feed the mimic a bodhran, a fiddle, a flute, a harp, a piano, a gittern, a hammered dulcimer, and a set of bagpipes. 

The mimic gets sick and turns green and shape changes to look like a trash can then from all that at once and lets out a giant bletch that is suddenly stopped by an egg that shoots out and hatches into a cockatrice 

i shove that last poor sap to throw something inside the mimic in the mimics mouth

The mimic coughs out the next poor sap to throw something inside the mimic.(I.e. Me)

I feed it the question to life, the universe, and everything.

The mimic spits out a pocket calculator and a note saying, "Do your own math!" 

I feed the mimic South End. 

The mimic spits out a compass.

I feed the mimic a black hole.

The mimic spits out a warrant for your arrest for attempted murder by black hole. 

I feed the mimic a Colt Peacemaker. 

The mimic spits out a horse that wants to make peace with everyone.

I force several hundred tons of creamed corn down the mimic's throat, accidentally feeding it three of the fingers on my left hand, and one on my right.

Oops accidentally posted twice

The mimic turns the shade of rotted wood and projectile vomits the creamed corn on everyone within 10 ft, farts, and promptly passes out.

I feed the mimic the wise old man from up the hill

The mimic spits out a magic demon slaying long sword that give you advantage on wisdom checks.

I feed the mimic a box of BSA and Eagle Scout stuff.

It spits out an American flag. Not that anyone knows what America is.

I red the mimic the sword from GreySky's post

The mimic spits up a book on magic swords, and magic reading glasses with shield shaped lenses and sword shaped temples. They magically keep your place with an invisible mark that can been seen while wearing them, and let you read all languages.

I feed the mimic a bunch of vinyl records and a turntable used for raves.

The mimic spits out a bearded hipster, with an ipad, who calls himself a DJ, but only plays songs everyone has heard a million times, and has become sick to death of.

I feed the mimic the hipster's beard.

The mimic grows a hipster beard.

I feed the mimic my toes.

The mimic makes toe jam that smells strong enough to permanently remove all the hair from it's body.

I feed the mimic a Sega Dreamcast with a bunch of games including Phantasy Star Online Ver.2, Rival Schools, Soul Caliber, Jet Set Radio, Cannon Spike and Rez.

The mimic spits out a taxi driven by a whip-sword wielding, jet-pack wearing Sonic the Hedgehog.

I feed the mimic Comic Book Guy and his Klingon dictionary.