The mimic spits out a magic 10 in 1 board game where players are shrunk to become there own player piece as long as they are willing to play (the secret 11th bonus game is a fully stocked pub and inn where simple games like chess, checkers, and 3 dragon ante can be played. To unlock and access this maybe you have to play through all the games with someone who doesn't know about the secret and plays willingly within 24 hours).

I feed the mimic a pair of adamantine revolvers  with magic weapon cast on each one (each is loaded 6 silver bullets), and a mithral shotgun gunblade (shotgun + long sword= ouch) also with magic weapon cast on it.  (I am left wondering if the mimic becomes like one in Enter The Gungeon and or one from the souls series).

In response, the mimic spits out a couple of cool looking samurai swords, but when you pick them up, they light on fire! They deal an extra 2d6 fire damage, but 1d4 fire damage to the wielder each round.

I feed the mimic my favorite hunk of enchanted cheese.

The mimic spits out a plate of raindrop shaped jello shots.

I drop in a book containing the complete works of H.P. Lovecraft (including his short stories and other articles).

The mimic ponders the existence of Cthulu as it absent mindedly nibbles on the end of cigar.

I feed the mimic a bran muffin.

The mimic reviews your character sheet and decides it's a bit overpowered before devouring it. It spits out a revised, and heavily nerfed copy of your character sheet.

I feed the mimic the sentient fart cloud mentioned earlier.

The mimic takes a deep breath and holds that fart like a bong hit then exhales a sentient ninja toaster with +3 bladed throwing toast and burnt crumbs smoke cloud 

by giving the mimic the blackest gift of all (NOTHING) the mimic spits out a small black box; inside is a list of the mimics favorite metal bands and songs.

I feed the mimic a few small statues of Anubis, Anput, Kebechet, some guitar picks made from metal, a necklace with the grim reaper on it, and an two elder signs (the original carved into star stone (Asterism) and the modern one on a necklace).

The mimic is slightly put off by how devoted you are to the genre, as it really only listens to that music in the car (do mimics have cars?). It tentatively eats your gift and vomits up an infant elder god. You go mad from the very sight of it and begin clawing at your eyes.

I feed the mimic my resume, I really need this job. ( I feel like this one has been done already, but I'm not going to bother to check.)

there just so happens to be an opening to keep track of and filter though everyone that comes to the mimic and what they bring so that they don't bring anything too useless or tasteless. Any overtime doesn't pay as well as some other jobs but the contract says the mimic and it's friends won't eat you.

I feed the mimic a bunch of seasons of Supernatural, Grimm, Invader Zim, and Samurai Jack.

The mimic doesn't accept it, in a strange turn of events. It turns out mimics don't, in fact, Love Love Teriyaki™. Instead it spits out a packet of soy sauce and urges you politely to have the meal.

I feed the mimic the complete Marvin Gaye Discography.

The mimic consumes your Marvin Gaye discography and spits out a gramophone.

I feed the mimic the collected works of Stephen King.

(I wonder if those are the revolvers I gave it) The mimic spits out a Nintendo Switch, and a few Nintendo gift cards.

I feed the mimic a steak dinner along with some Guinness.

The mimic coughs up a table and some chairs and even some nice lighting to set the mood. He then hobbles over and takes a seat. Casually taking small bites of his steak as he wants to savor the flavor and occasionally sipping his Guinness. Once done he barfs up several delicious desert plates for anyone who wishes to join him at the table.

I join the mimic at the table and offer him one slice of cake :) 

The mimic eats the cake, enjoying the taste, before he realizes that he is allergic to gluten. The mimic vomits out the cake, its acidic bile burning the tableset and all those in range. 

I give the mimic some Tums.

The mimic belches loudly it's tongue wriggling in the air a la Homer Simpson.  A moment later a slow smile spreads across it's wide jaws.

I feed the mimic my 3rd ed Tenser's Floating Disc

The mimic hovers a bit before spitting out a spinfusor disc (Tribes), wrecking everything in it's path.

I feed the mimic a McChicken (un-molested)