I'm a list guy so here's a list of ways to spice up encounters both violent and boring (I mean social).
As far as dungeon design mixing the familiar and fantastical for flavorful or mechanical effects keeps player engaged and excited about what basically amounts to a series of boxes full of blood, treasure, and XP. As with all things RP Immersion is key.
-In a natural/cave dungeon spice things up with a peppering (double food metaphor?! I must be hungry) of interesting area features. Such as an underground stream in a natural cave that goes to additional rooms (maybe a monster flees into it luring the players into a trap) or humming stagtmites/stagtites that mask the sound of the party (or monsters) and become louder when struck. Google some weird world or crazy nature facts for inspiration.
-In an abandoned/ancient type dungeon play up the age. If a trap fails to trigger after the PC's fail to disarm it they'll really feel the age of the place. Make them concerned about ceiling collapses or rickety stair cases. Use molds, oozes and slimes. They should feel as uncomfortable every step as you would walking through an abandoned building that's gone to pot.
-In an "evil fortress" active defense should be more than a box full of orcs. Favorites I've used are an Enter the Dragon mirror maze and a hidden machine that constantly rearranges the walls of the dungeon. Consider crawl spaces and murder holes. A person is unlikely to have a bunch of traps in his/her own home but plenty of ways to defend it. Make the players feel like they're fighting the building less than it's residents.
For social stuff always remember that NPCs are people and most of them have more in common with the players than the players have in common with their murder hobo alter egos.
-If the PC's interact with an NPC have them meet someone who knows him/her in the next town. Keep this up and they'll start to feel an investment in the world.
-If a social encounter turns violent try to remember how normal people react to super powered kill gods (I mean adventurers). Even a Lvl 1 mage casting burning hands would strike awe and terror into a drunk bully or mugger. Flight and begging make a lot more sense than good ole boys from town fighting to the death against impossible odds. Keep them feeling like people and the PCs are more likely to treat them like that and therefore become more immersed.
-PCs are RICH. Everything should feel like a scam or a too good to be true party while the PCs are camped out in town. The Mongoose Conan even had a mechanic for the PCs loosing half their wealth every week they were just hanging out in town due to poor decision making. This should prod them into investing meaningfully in the world.
Besides that listen to your players and try to give them what they want in your encounters. If one player walks away remembering an encounter forever every time you run you're doing your job perfectly.
One more thing, for random monsters the old 2nd ed Monster Manual ecology section can spice up/explain why you met monsters and even lead to whole adventures.
As far as dungeon design mixing the familiar and fantastical for flavorful or mechanical effects keeps player engaged and excited about what basically amounts to a series of boxes full of blood, treasure, and XP. As with all things RP Immersion is key.
-In a natural/cave dungeon spice things up with a peppering (double food metaphor?! I must be hungry) of interesting area features. Such as an underground stream in a natural cave that goes to additional rooms (maybe a monster flees into it luring the players into a trap) or humming stagtmites/stagtites that mask the sound of the party (or monsters) and become louder when struck. Google some weird world or crazy nature facts for inspiration.
-In an abandoned/ancient type dungeon play up the age. If a trap fails to trigger after the PC's fail to disarm it they'll really feel the age of the place. Make them concerned about ceiling collapses or rickety stair cases. Use molds, oozes and slimes. They should feel as uncomfortable every step as you would walking through an abandoned building that's gone to pot.
-In an "evil fortress" active defense should be more than a box full of orcs. Favorites I've used are an Enter the Dragon mirror maze and a hidden machine that constantly rearranges the walls of the dungeon. Consider crawl spaces and murder holes. A person is unlikely to have a bunch of traps in his/her own home but plenty of ways to defend it. Make the players feel like they're fighting the building less than it's residents.
For social stuff always remember that NPCs are people and most of them have more in common with the players than the players have in common with their murder hobo alter egos.
-If the PC's interact with an NPC have them meet someone who knows him/her in the next town. Keep this up and they'll start to feel an investment in the world.
-If a social encounter turns violent try to remember how normal people react to super powered kill gods (I mean adventurers). Even a Lvl 1 mage casting burning hands would strike awe and terror into a drunk bully or mugger. Flight and begging make a lot more sense than good ole boys from town fighting to the death against impossible odds. Keep them feeling like people and the PCs are more likely to treat them like that and therefore become more immersed.
-PCs are RICH. Everything should feel like a scam or a too good to be true party while the PCs are camped out in town. The Mongoose Conan even had a mechanic for the PCs loosing half their wealth every week they were just hanging out in town due to poor decision making. This should prod them into investing meaningfully in the world.
Besides that listen to your players and try to give them what they want in your encounters. If one player walks away remembering an encounter forever every time you run you're doing your job perfectly.
One more thing, for random monsters the old 2nd ed Monster Manual ecology section can spice up/explain why you met monsters and even lead to whole adventures.