How do you guys handle players with really high passive perception? I have one that got the Observant feat and high Perception already, giving them 21 Passive Perception at level 1... How the hell do I deal with that!? They spot everything at all times...
Easy - every task is a trade-off.
While adventuring, the players choose ONE general task they are undertaking as they do their thing. That might include Keeping Watch (for monsters and traps), Drawing a Map, Foraging, Tracking, Navigating, Searching for Secret Doors, etc. When you attempt one of these tasks, you can't do another at the same time (unless you're a ranger in favored terrain). Because this is a task that is performed repeatedly, the DM can use a passive check to resolve the outcome, when that outcome is uncertain.
So if your Observant PC wants to focus on avoiding being surprised by monsters, then he or she must Keep Watch. That means no drawing of maps or foraging or finding secret doors or whatever other task that might reasonably distract from Keeping Watch. If instead the Observant PC wants to Search for Secret Doors in the dungeon, then that means if lurking monsters come calling, he or she is automatically surprised since the character is not Keeping Watch. (Again, assuming the character is not a ranger in favored terrain.)
Further, you have to be in the position to notice things. If the party is approaching a hidden trap, for example, the Observant PC can only have a chance of spotting it IF the PC is both Keeping Watch and in the front rank. This means you not only have a trade-off, but you have to put yourself at some risk, too, by being in the front of the party.
In all likelihood, this will still mean the Observant PC will notice all the things he or she is focusing on. But the character can't focus on everything. There's a meaningful choice for him or her to make here that exists in the context of the setting rather than just when building the character.