The trifecta of pain aside (Hell, Silent tower, Shadow Plane)....there are plenty of good places to explore.
The siege encampment is difficult with lots of stuff to see and find there. There is an in quest puzzle you do solely with observation so anyone that looks around enough can figure it out. Going to be hard to solo.
I haven't prodded around there too much myself but Verathi's new area south of seantryn modan that replaced Haradan woods is supposed to be excellent. Lots of hidden nooks, crannies, and places to map out there. Also, there is a very puzzlish trap part of it supposedly that I want to go mess with at some point.
If you are in the level ranges of 30-35 you can check out the sub hero explore area that replaced the sirines northeast of seantryn modan. Its extremely difficult fight wise and puzzle wise, its cost more than a few people con points.
Lost elven vaults is a fairly easy place to explore, with a moderate level of difficulty if you are hero and a few dangerous traps, but nothing insane imo. If you haven't explored there, go explore there alone, you may enjoy it.
Not so much for puzzles and whatnot here, but Whistlewood has some very, very cool lore laying around if you like to read in area stuff like that and get a feel for why an area is why it is.
Octogonal tower is fairly straightforward, with a lot of cool, cool in area lore to it. Other than finding some keys that some obvious mobs around the area drop, its basically a smash and grab with difficult fights.
If you don't know the trothon riddles, and haven't been there, those are really fun to try and learn on your own.
Surprisingly, Hamsah Mutazz has some really cool lore, in area secrets, and quests. A fun place to snoop around.
If you can get a group, Dragon lairs are a fairly straight forward smash and grab. Also with a group, Dragon tower ruins is sort of the same but with some hidden passageways and containers and stuff, slightly more explorey than the dragon lairs.
Aran'Gird is a fun zone, has an extra area attached to it that is its own sub area of mineshafts. Mostly a smash and grab, with a maze and a hidden mob.
The aboleths can be fun to figure out, dangerous so I would take at least a friend or two. A very cool in area dragon freeing quest there.
There is a lot more, but that should keep you busy for quite a while. Feel free to message me if you want me to rummage up more areas I think would be challenging for exploring.
