Each landmark is different. Some have quest lines and won’t unlock until those quests are complete.
Crypt of the Revenants can be completed proactively by looking for a quest in the nearby cliffs. A band of orcs, one of whom is missing an ear will get you started. I believe the landmarks in Regan Gor and the Tuscari Sea also have questlines.
Silver Drift Hollow requires you to max out the relations with the town. Then do a small quest. There are two (Sevenkeeps and the Forge of Uram Gor) other landmarks that unlock this way.
Some landmarks are easy (just defeat the guardians).
And there are the dragonbones in the Misty Coast, which just requires you to wait after discovering it…