Another think I would enjoy doing was push my players out of their comfort levels. A lot of times that involved twisting around monsters or using spells in ways they wouldn’t expect…
I remember one quest the players heard about a village being enslaved by a necromancer. Upon arrival they found that the major of the village was a necromancer but used his undead minions as farm hands to help out the farmers. As many of them had been kidnapped or out right killed by other “real” villains.
After getting my MM2 one of the monsters I would use a lot really wasn’t a monster at all but a monster template. I would use this template for many monsters. Many of them weren’t hostile at all, but it was a cool way to remember a ncp. “You mean Ricky, the ½ stone golem bar keep?” I would use that Half Golem template a lot like you see automail used in the Fullmetal Alchemist.
Doing this did two things for me. One made some NCPs more memorable. Some Dms would do this with personality/job alone but I think making the NCPs a possible threat does a better job. Players will keep a running list of things they are afraid of in mind, boring plot points not so much… And two lets me use that NPC later as a threat. Making an encounter out of a early plot point can change the players view of the world. A major shake really makes them think….