Spellcasting is #1, obviously not because it's intellectually difficult to understand, but it's hard to remember what spells any given monster has, even if you prep really well, and sometimes it's hard to remember what those spells do. More than once I've been about to have a monster cast a spell, when I realized it already had a concentration spell up, for example.
Spellcasting for monsters in 5E is way too widespread. It needs to be cut down. I get that it was some sort of throwback "this isn't 4E" thing, but guess what? 4E did a better job here, 5E - learn from it. Some monsters having spells is fine, but too many have them in 5E, and they're too fiddly to use.
After that definitely reactions which only do something minor and defensive.
I mean, a monster can have a 20 intelligence, how much can I "cheat" to emulate that they're thinking 5 moves ahead even though I can't? A truly intelligent monster should have contingencies and plans particularly if they are forewarned and have had time to prep.
I think this is a misapprehension. Does an INT 20 PC have layers of contingencies and plans? Do really smart people IRL have layers of contingencies and plans? I say not to both. This is a weird trope. INT 20 does not automatically mean "Xanatos from Gargoyles". That's a personality AND intelligence thing. Not every smart monster will have great plans. Not every smart monster will be a tactical genius. I'm pretty sure Einstein might have screwed up big in a fight.
So the real answer is don't try to play them like that unless they're the rare sort who is like that (the Xanatoses of your world).
Yes, if they're forewarned and have time to prep, they may have more fallbacks/plans, but then it's all about intelligence in the other sense - i.e. knowledge of your enemy. How much do they really know about the PCs and their methods and why they're here and so on? If it's very little, but enough to be scared, if the monster has any common sense at all, they're going to leave, perhaps leaving behind some sort of way to observe. Or they'll mount an ambush that hopefully leaves them relatively unegaged and able to flee. Or if they're arrogant, they'll do something dumb like trying to overawe the PCs. But you look at the whole of the personality. The idea that 200 IQ or whatever means they'll always have good plans is a complete denial of the reality of smart people.