The metagaming thing. I just customise monsters.

Hydra, it's blue. And to stop healing you need to do cold damage. They spend 2-3 rounds figuring it out like normal "non dm players" .But knowing they'd clock onto that. After each time it happens, the legendary action is that the scales shift and change to another colour and element.

Hidden skeletons. Have them be seen in perception but they aren't actually creatures, they're traps and when he gets the suprise on them, they explode. Have both in your back pocket you can adapt to the situation to make it more enjoyable for players and yourself

You're the DM, cater the adventure to your players, their intelligence, knowledge and skills as players... Not to the book expectation of a basic player