Nitroglycerin Monsters

And he looked just like a
harmless wabbit.

Some monsters make good traps for players stuck in a rut of hack and slash. I think of these as the "Nitro Monsters": generally safe if handled with care and devastating if knocked about.

Giants are a good example. Although many evil giants are, well, evil they have a background (in fairytales) which suggests that they don't have to be mindlessly aggressive all the time. Three frost giants extracting tolls on a mountain pass may well know that if they cause too much trouble then something will be done about them. A party who is prepared to simply pay the toll can pass unhindered while one that wants to make a fight of it will face needless risk to life and limb.

Good-aligned dragons are another example, as are neutral monsters like gynosphinxes and even elementals, but for experienced players one-off weirdo monsters that the players have not encountered before or have any reason to have preconceived notions about are best. Classed NPC characters are probably the most effective as traps as they inherently have no specific danger level associated with them, but they are perhaps too well disguised to really play the part. Ideally, a party who gets their arses kicked by such an encounter should know it's their own fault without being told (so Bugs Bunny is perhaps a bit too evil too).

As mentioned previously, AD&D is a game wherein resources are the key to success and while a pit trap or such here and there can certainly cause attrition of those resources they are rather passive. But the Nitro Monster is both a trap for the incautious and a roleplaying opportunity in one. There's even the chance that careful dealing will result in the addition of some extra muscle to the party at least for a short time, so they potentially can add a lot more to a play session than the generic types of traps and tricks.

"Kill some adventurers?
I've got a budget speech to prepare!"

The other thing that's good about Nitro Monsters is that they can be used at any level. If you have a vampire running a small country somewhere then PCs can pass through that country, perhaps even dealing with the vampire, even at 1st level. The vampire's ruling a country, not mugging passersby, and has other things to think about than simply slaughtering any stranger.

This allows even low level parties to interact with some of the "big hitters" of the fantasy world instead of always dealing with goblins and the occasional orc; and lets the DM set up hooks for future adventures when the characters are high level enough to think about the vampire and the frost giants they've met over the years.

And if the first level party decides to attack the Nitro Monster, it's their own fault if it goes "Boom".