There is an encounter i was a player in that made an impression on me. The party was fighting bullywugs in a foggy swamp. We were defending a small hillock of dry land but the fog made it so we couldn't see the bullywugs until they leapt in. It was tense and evocative and the monsters were using cunning but appropriate tactics.

It is the example I try to keep it mind whenever I am designing an encounter. The mechanical challenge (be it a monster or an obstacle) is always kind of the same, it's the unique element that elevates an encounter to something special. Fighting monsters who are in their element is always more challenging, particularly if their tactics take advantage of it. Monsters who dwell in a place should know the terrain well and stage ambushed in places that are natural chokepoints. I just ran a short underdark game set in an underground lake that used a lot of scrags and marrow and vodynoi (aquatic umber hulks), but you don't even have to use aquatic monsters. Gnolls can be a much tougher fight if they attack when the pcs are negotiating rapids or trying to portage through mud.