Exploring The ruins of Gladen, close to the city of Cosnaught, is a more deadly encounter well no Minotaur, and no Medusa, but more deadly because there are no kobolds and it contains a community of gobinoids, like goblins and kobolds and bugbears.  Fun.  Also the lost treasures of Relic and his planning room.  There are rumours that there are many false chambers within his palace that no one found after his reign nor since the city collapsed.  And if that were not enough, his stash of treasure is thought to contain plans of the major cities and the palaces and treasure vaults of many of the cities of his day.  Good place to introduce a couple manuals of stat increase, and a few treasure maps that are a couple hundred years old.  Perhaps one of the maps leads to a hidden stronghold in the mountains that Relic used as a cache— cool Jules Verne shit like using astronomy stuff like from the perch of he mountain with three boulders at the summit, stand by the light the light of the Darkest Darkday in High Summer, the peak in the Drake points the way at noon. 

So the Darkest Darkday in the high summer corresponds to the time when the blue sun is eclipsed by the red sun, while it eclipses the green sun too, the sky becomes less deep blue in this week and takes on a more green hue.  When the Darkday occurs the sky becomes like night and the land is cast in red light.  And the stars are visible.  Very eerie.  Add that you need to have the light of the Red sun in darkness to show the correct way into the hideout meaning that you have to do it on one specific day of the year or have a priest or the red sun to cast a light spell at night to find the correct entrance.  Or a priest of the Hidden Goddess, because it is hidden and coolly done at that.  So the question is from the three boulder mountain summit which mountain is in the constellation of  Gull, Drake, Serpent, Rabbit, Hearth, or Hammer; it is a directional question.  Then the next part is a question of where the shadow of the peak will reach on the the Darkest Darkday of the year on High Summer.  That is at noon the peaks shadow will cross a cave mouth, or it will point directly at the mouth of the cave.  If they do it at noon on the correct day they will see a sparkle, but that is a special case, not very likely.  But if they do the math correctly, make a astronomy/astrology check and the closer to 20 gives a more accurate result.  Each point of failure is off by a kilometer, every point over a 20 increases the accuracy.  If they use a light spell from a priest of either the red sun or the blue sun they get a bonus to find the door, as it reflects that pure light magically.

Complication, about a century some enterprising group cleared the boulders off the mountain, because it interfered with something.  They will need to find someone who remembers which mountain had the boulders on it.  There are two possibilities.  There is a enclave of Grey Elves in the region and they know of the mountain in question.  There is also a small isolated Stone Giant clan that know about it.  If they find the correct mountain at the summit they will discover an entrance to the Dwarven Empire, a crew masons removed the boulders and turned them into building material, as the stone was of superior quality, but the local dwarves are used to looking at the inside of a mountain and don't concern themselves with the outside.  Hell, the players may have climbed the mountain in question, but the dwarves moved there fifty years ago and know nothing about the mountain.  

Additionally there is a a group of outcaste Stone Giants in the region of the cave entrance and a Dragon.  There is always a dragon in the region, probably red because of the mountains, but there is a substantial forest at the base of the mountains, so it could be green, appropriate challenge rating.  It is a smart creature it may bargain with the characters if they reveal what they are looking for, the dragon may remember the mountain with the three boulders too.  There might be two dragons in the area if the characters are sufficiently low level.  The Green living in a different biosphere than the Red, and sufficient distance.  Either will bargain for the death of the other.  The green will bargain in good faith, requesting first pick of the treasure and such.  The red would betray the characters at best opportunity, preferably after they defeat the Green.  

That is one treasure map.  There will be others.  Perhaps one of the treasure vault of the emperor of Gutral Mastekeena.  Why would Heroic characters seek to break into the treasure rooms of emperor?  It might be that the Emperor has let the people know that it has something that they need to defeat the Dragon, perhaps a manuscript speaking about the Orb of Dragonkind.  The emperor is willing to part with it for a price, a thousand Dwarf trained artisans to be made personal slaves of the emperor.  The PCs have a clear planned path to the vault thought the palace, but it is through the palace!!  If they manage to scout out the city they will spot an easier route, through the sewers.  The sewers have their own issues, like the chambers with oytoghs and wandering undead.  There sewer entrance is quite close by the not quite royal crypts; the royals are buried in a more regal crypt outside the city, but the lesser royalty and royal servants are in the crypt and they are part of a elaborate trap, after the more mundane traps like dart throwers and swinging blade traps and blade barrier type traps.  The last is a block trap which opens up into the crypt, if they survive their saving throws.  The crypts are filled with a combination of lesser mummies and wights and ghouls.  As soon as the PCs activate the last trap and enter the tombs an alarm goes off in the Palace and guards begin to move to engage the PCs.  The guards vanguard includes six Shield Guardians and the vault includes stone golems.  The guardians will not enter the crypt unless the PCs steal some of the greater treasures.  A single golem will chase the PCs down if they take the manuscript, though.  If they get into the sewers again they will notice that exits to the city are heavily guarded including the entrance they used.  If they stay in the sewer, genies will chase them down and report their location.  The only way they can leave is if they have planned an escape route that leads into the river and out.  Breathing underwater and all that in the plan.