In my new 5e Stonehell campaign, as GM I've taken to rolling for spot difficulty vs the PC's Passive Perception.
Easy - d20
Moderate - d20+5
Hard - d20+10
Example: So the players have their highest PP PC the cat girl Bright Star of the East on point with PP 16. She enters a room with a secret door, I roll d20+10 (Hard DC) for the difficulty of spotting it, get 6 and say "You notice a concealed door..."
This has worked really really well and is a case where not having the DCs listed in advance has helped a lot.
