For a one shot, it may be a good idea to browse the monsters in the monster manual.
Some monsters are a mini campaign all by themselves.
Take Aboleth for example. Slimy eldritch fish monster. It has the ability to enthrall folks and turn them into aquatic slaves. Saving someone from the clutches of an aboleth could make a great oneshot. No need to take on the aboleth directly. Party could just be there to save a friend. Potential for TPK (or total party enslavement) is high, so great one shot.
A troll menacing a village also makes a great one shot. Troll can't be killed with normal weapons. So Party would need to be creative to take it out. Especially if they're low level. Troll comes every month to collect it's tribute - but this time the village is ready for it! (or ready to tick it off enough to slaughter everyone). Adventure is about setting up a fire based ambush for the the troll.
Sometimes the simple stuff is the best stuff.
... Play'n the FLUTE while I LOOT dat dead kid's BODEHY!