Gobin Fortress - here the goblins would have a good numbers advantage for challenge. But somehow a huge fight in Gut's or Dror Ragzlin's hall doesn't even alert the closest goblins behind a corner. You can basically march in and clear one room at a time, and teleport out to Long Rest whenever. It's so easy that it really doesn't encourage infiltrating the place as a True Soul or using a Drow disguise, which could otherwise be really exciting knowing the entire fort will aggro if you fail some check.
Disable Fast Travel and make Goblins more alert to fighting. Reinforcements swarming in from behind to surround the party should be the flavor here.
Phase Spider Lair - just double or triple the numbers of Ettercaps and Phase Spiders. Remove the spiders' spit ability and make them attack in melee where they can actually hurt you. Add roaming enemy groups that eventually find you even if you don't move and join in when battle starts anywhere. So the party eventually gets surrounded unless they're careful. Disable Fast Travel.
The Underdark - So safe and boring! Disable Fast Travel and add roaming monsters that can appear anytime anywhere. Make the player scared to leave the safety of the Myconid Village or the Selunite Fortress.
Speaking of the Underdark, what happened to dark areas? Low light doesn't seem to be an issue anywhere anymore. Humans and Gith can see perfectly in the "darkest" areas and you never need torches or light to counter a low light Disadvantage. Not only a huge atmosphere fail, but also another thing that makes the game a lot easier.
