I'm under the impression there might be a secret in the Hot Spring in the later half of the Mountain Peak stage, just REALLY not sure how to trigger it. Ghost girl is still a mystery to everyone except the creator, and the Slime handjob might be triggered when your character is sitting down in a chair. (You can only sit on specific chairs/benches or chairs spawned in when having sex with the Devil NPC)
Not sure if you caught it but you can also 'Win' panties from Iris and Lisa after you do their rematch. You get nothing from rematching the first three bosses, even though they are harder during the Rematch (Alaruna's apples drop more frequently, Undine's water starts as an lust trap instead of becoming one in the 2nd phase, and Golden Fox is more aggressive with her attacks. On top of all the new attacks they have, Golden Fox might be the hardest if you're not gonna try and cheese her with invincibility attacks like Golden Fox Illusion or Mimic Box while sitting in a Succubus Sanctuary)
There are two very hidden items, one in the first 'real' combat area with the slimes behind the first tree and the other is in the Mountai Rest area or the 'Lab' behind a wall that makes the room look 'foggy'. That said, I have all the items that the picture book allows me to see so other than panties, you can get all the equip-able stuff through the shop.