So after a series of unfortunate accidents one of my players (the pugilist - homebrewed monk) got sent to the Fey realm. (Every action leading up to it was caused by the player) The rest of the party desided to follow, but idk what I should have happen while they are there. I have a npc/potential party member ready for later but the setup for that will be too much for them for at least 2 levels. I need some advice for ideas of what can happen in the Fey Realm (There main mission will likely be to escape the realm once they enter it, and it wss established that way they are going in is only one way)

The most fun I have with letting people into the Feywyld accidentally is to make it purely surreal.

As your eyes adjust to the light you are struck by the beauty of this place, you know it's not anywhere located in the world you know. There are vibrant colors everywhere, like someone has taken the filter off of your eyes. The giant, table sized sunflowers are a bright yellow, an entire field facing directly at the sun above. That sky is a crisp blue, the starch white clouds slowly moving along on a breeze. The smells of sweet blossoms is also met by the most refreshing scent of clean air, as if there were no impurities at all. As if that weren't enough, you swear there are creatures all about you, but they seem to be just out of sight, watching, waiting, for what? You sense no malice, yet, only a strange and foreign curiosity, as though these creatures are as surprised by you as you are of the world you have just found yourself in.

Think about Alice when she first entered Wonderland, or Lucy first entering Narnia, there's this sense of wonder and awe, try to tap into that. Give them descriptions that elicit the idea of vibrant colors, foreign plants, strange noises, and foreign imagery. Impress upon them the strange way the world reacts to them, plants moving in odd ways, smells that don't quite match up with what they're seeing, creatures that appear just at the edge of their sight. If, an when, they meet a denizen of the Feywyld have it be as strange for the native as it is for the group.

I'd also have the world seem intimidating and dangerous, this is an unexlored world of strange and fre magic. Have plants that try to lure them into their maw, shoot needles at them, vines that flow and follow them like snakes. Have the creatures of the area seem wary of them, distrustful, and somewhat hostile. Trick them with the wily ones, have them led into traps of dangerous situations by a creature that is assumed to be friendly by default.

Make their goal become leaving, and finding any reason to do so. Then give them some entertaining ways to get out, from eating a strange concoction to making deals with the Seeley Court. 

Big fan of the above post, just a surreal, beautiful, but potentially dangerous terrain. I'd throw in a good amount of Plant perils, giant carnivorous fly traps or sweet smelling pitcher plants that can lure in the adventurers and even be able to Charm. Vines that grapple and whisk someone high into the trees, then dropped for damage. Poison thorns, etc. I'd also imagine a huge potential for misunderstandings due to language barriers and cultural differences, forcing the players into thinking differently to get out of trouble. I'd love an update on your decisions upon your next session.

I would suggest googling fey legends / fairy legends and the like. Long story short the fey storys usually start with wonder if they feature wonder at all and end in horror or regret as the mortals are played with by their hosts like a cat plays with mice. The fey in those stories dont see humans as anything as something thats fun to watch bleed.

Jim Butchers The Dresden Files series features some fantastic takes on fey legends, Laurrel K Hamiltons Meredith Gentry series dives into sex(be warned) but around that adds some great motivators for why Fey steal mortal children and hate mortals at the same time.

King Arthur the roleplaying wargame (computer game, NeocoreGames developer, Paradox interactive  publisher) features fey and they appear as alien, hostile and beautiful and (for me) shows perfectly that the only difference between the courts in how their interaction will proceed with mortals is whether you die warm or cold.

A practical guide to evil -webcomic (after some time) introduces a brutal cruel pointless war of fey on fey with catspaws dying for them for no good reason.

Finally the dmg mentions the feywyld briefly. It even mentions the cruelest of the fates the fey inflict but not why. When you enter the fey your sense of time is irrelevant as the fey realms care not for the laws of man or science or nature. Whether you spend minutes or years in the fey realms the amount of time passed in the mortal realm could have barely passed at all or every last one of your relatives could have long died in the centuries you were presumed lost.

Have fun with that :)

Also, look into the Seelie and Unseelie courts. Two factions that are wildly different in the Fey.

Ah I didnt mention courts because when your running it you have to makes a decision for yourself on whether you want them / what you want them to be (seasonal, light and dark beautiful and deformed, river stream and field, Slaugh etc) The courts may not even be relevant to you depending on if you want the party to escape the fey lands (the references I mentioned before... yeesh no politics thank you, its fatal) Alternatively it might be crucial that they make a deal.

Here's a point most materials agree on - you should never make a deal with the fey it sucks, and the second point explaining why the first point is often ignored - the fey never lie, to do that physically sickens and can kill them. But they will speak the truth in a way that will make sure what you know.. is only what you think you know. Also if you lie to the fey they take it as an insult worth killing over. I say most materials because the DnD fey have not really been given their own source / lore tome ever from TSR or WotC as far as I know and its never been mentioned what they can and cant do. Make your own rules up in that regard.

Dont be afraid to use non fey creatures as fey - notably the harpy as luring temptress that kills those it loves even as it sheds tears works well as do Ogres as the fey version of goblins (they are nasty) the goblin court of fey isnt the goblins you know. Another gold standard of fey are trolls, yup trolls, you might want to change up the regeneration bane to cold iron (a fey weakness in legend or sometimes just iron - but that seems cruel as cold iron isnt a thing anymore.

Try throwing in some encounters with exotic wild animals (I.E: Blink dogs) or just ordinary Gnomes / Satyrs / Druids, etc.

You could also use their behavior to show how deadly the landscape is, I.E: A Gnome NPC warning the party to clear out because it wouldnt be safe soon, or a bunch of Blink Dogs teleporting away nearly simultaneously. 

Check out what Matt Mercer did with the Fey.  He took sprites and pixies, flip-flopped their alignments, and made them way more powerful. A cool idea if you have players that like to meta.