The mimic promptly spits me back out.  Apparently, I taste terrible. 

I feed Mightylemur to the mimic. 

The mimic swallows Mightylemur whole. It then burps out a few tufts of fur. Apparently it thinks lemurs are quite tasty.

I feed the mimic a participation trophy from a youth sports team.

The mimic spits out a dead youth sports team. 

I feed the mimic a flame tongue.

The mimic spits out a male elf, the old wielder of that flametounge.

I feed the mimic a Magic the Gathering booster pack.

The mimic spits out a mountain, which crushes you. 

I feed the mimic a boulder. 

Quote from SeargentBobXIX >>

The mimic spits out a male elf, the old wielder of that flametounge.

I feed the mimic a Magic the Gathering booster pack.

The mimic spits out a sorcerer.  The sorcerer calls down a meteor swarm.  The mimic emerges unscathed from the crater. 

I feed the mimic some whale meat and the remains of a petunia. 

A grotesque ooze, bubbling and boiling, pours out of the mimic with a faint smell of the sea and the fields.

I feed the mimic my ring of sanity

The mimic spits out a spinning red ring of doom. 

I feed the mimic D&D Beyond's character creator. 

The mimic spits out a character sheet of a 1st level aarakocra wizard named Hunoar that knows Acid splash, chill touch, control flames, absorb elements, burning hands, catapult, charm person, color spray, it also knows alarm as a ritual.

I feed the mimic Hunoar's character sheet.

The mimic spits out Hunoar. 

I feed the mimic Hunoar's spellbook. 

The mimic spits out a spell scroll with the spell mirror image written on it.

I feed the mimic an immovable rod.

The mimic spits out a magic anvil of gravity. 

I feed gravity to the mimic. 

After somehow feeding the mimic gravity, you are no longer held to the ground and float into the sky. The mimic is okay though.

I feed the mimic Wednesday.

The mimic spits her back out, along with her rapier.  She runs you through with it. 

I feed the mimic your corpse. 

The mimic spits undead me back out, where I then do the same to you.

I feed the mimic your corpse.

The mimic spits out rubber and glue and whatever anyone does to anyone else is done back to elementary school you. 

I feed the mimic a spinach salad with enchanted void dressing. 

The mimic licks its lips and spits out Tinderstrike. 

I feed the mimic an iron flask containing Maegera the Dawn Titan. 

The mimic swallows your iron flask, and burps up a Swarm of Spiders

I feed the mimic a small child wearing an iron man mask

Robert Downey Junior himself emerges from the mimic's maw. He isn't spit out, he just sorta pushes himself up and out. He looks a little weary as he looks around the room before setting eyes on you. "Bathroom?" he half mutters to you while pointing to the door. You have know idea if there is a bathroom through there, but you're just as shocked as RDJ at this point and nod slowly. Nodding, the A-list actor walks shakily to the door, turning to give one last bewildered look at the mimic before his dips through the threshold and closes the door behind him.

I feed the mimic half of a kitkat bar.