He can get annoying but Grohlvana's video on Skyrim's Hidden Treasures(?) is good for that kind of thing.

Delphine is more than she seems; she's not just an Innkeeper. Bet you didn't know that!

Go to Riften and do all of the fishing quests from Viriya and the other guy. So fun. 

Find poor Lucky Lorenz

You would love CamelWork's "Curating Curious Curiosities" series on YouTube.

Kagrenzel is far enough out of the way that it won't even show up on your compass unless you go out of your way to get close enough. Without the paid CC mods, there are no quests that point in that direction.

https://www.reddit.com/r/skyrim/comments/14iv6lh/ive_never_heard_of_this_place_ever_its_kinda_eerie/

I watched like 5 Youtube videos fir Shalidors maze and STILL couldn’t/cant find it lol

I had never found that one ruin you enter where you activate an orb, and then the floor drops out from under you and you fall deep into the abyss (like Gandalf and the Balrog).

That was on a recent session, it was cool as shit fighting my way back up the twisting walkways

I guess the lingering frost trolls dissuade most from exploring, also from what I recall going through the maze with Frostfall enabled was a fucking pain.

"mid" lol

It took so dang long to do this

There's a little "lover's campsite" set up just NW of Dawnstar on the coast. I only found it recently (as in the past year) or if I've found it before I never paid attention to what was going on. No one is there, but it tells a pretty obvious story. ;)

Also, here's a HUGE list of unmarked places of interest.

Does DB DLC count? If so then Kaarstags quest locations. Also just look around for interesting formations and things on map, grab a horse and go explore.

I know that outside Whiterun, in an indent in the mountain that holds it up, under Dragonsreach there's a small unmarked bandit hideout.

My friend is also fond of the chicken necromancer, up a hill near Ivarstead.

I found the fairy rings in Falkreath to be beautiful.

Hot springs between Riften and Windhelm. Got npcs just chillin in the water. But don't steal their gear. They don't like that shit.

When you are in the Twin Falls area, there’s a spot west of the Arkovian ruins, up the road and through some rocks and bramble that leads to a heroic chest, I never knew it was there until I spotted it by accident

IDK how many people just randomly find those when playing on their own but IMPO the "secret" optional areas with a boss in them are the coolest things in Grim Dawn, like the Arkovian Coliseum and the Corrupted Tomb

Discovering the Path of the Three on my own was pretty sick.

The shrine to Korvaak in Blood Grove

It's possible to see hidden paths if I rotate the camera to look directly at them. So I don't have to refer to a grimtools map whilst hunting for locations if I remember approximately where to look.

Not a secret per se, but there is an amazing lore bit hidden in a conversation with one of the Ebon Tower Curia scribes. When you talk to him, he mentions that he can't stand the Simulacrum era bureaucracy, because back then maps were issued with imperialized names for the towns and settlements in Tamriel, and even some of those "settlements" being nonexistent. This is by far one of the best ways to justify the "retcon" of the old Arena and Daggerfall lore.

There is a city full of daedra/ghosts a lof of them being npcs you can talk with.

Another good one is a certain Indoril town that got teleported into oblivion. You can actually visit that realm.

The armistice signed between Tiber Septim and Vivec is housed in Old Ebonheart but it is locked in a secure vault… Why not try to find it.

There's a 2nd era ruin where the dead start walking as you explore it. I think this was somewhere in the Amun Ashlands, but I'm not super sure. It's been a while since I found it.

Be wary! This is an agreement between two gods, things may not be as they seem.

If you look underwater in the bay east of Tel Branora, you will find a sunken Dunmer stronghold that holds a secret!

There is a hidden cave, off the map, in "The Battle of Ilum" fp, where you find a bunch of crates for the bonus quest and a frozen tauntaun. I think there may have been a skeleton in there as well, don't remember it that good.

I've got a character that I left standing on one of the highest buildings in the main city on Rishi. A friend and I scaled up from building to building by dueling/pushing and Sorc pulling to the highest point we could get to. Not necessarily hidden, but try with a friend to find weird places like that.

The little chamber on Korriban w/ Klor Slug is neat and a lot of people don't know about it.

There are lots of "weird" areas you aren't supposed to be! There are various ways and bugs to get on the outside of the Fleet, and you can strange places like the surface of CZ-198 or under your Yavin 4 / Manaan Stronghold.
I'd like to hear about more places like the ones you described though!

Section X on Belsavis was originally part of the Belsavis map, and twas connected to The Tomb region. The blocked path is pretty obvious if you're looking for it. Looks for Byond the Walls on YouTube by SWTOR Central.

Forelhost and the Dragonpriest Rahgot; make sure to read the books and pay attention to the environmental storytelling (mass graves and baby draugr wrappings, for instance).

Absolutely disquieting as hell. It really stuck with me. Frostlow is the most popular pick, but Forelhost is a different kind of nightmare.

Frostflow Lighthouse is a good one!

Riften Orphanage, no...not because our beloved old lady. Children in Skyrim are just a pain.

The Dwemer buildings surrounding Blackreach creeped me the hell out last time I played. Felt like something was behind my character at all times while I was exploring.

There's a spider den in the west of Eastmarch that is absolutely terrifying if you hate the creepy crawlies. It's hard to anticipate where they are coming from with the tunnels circling around you