Not sure how many are "secrets", but they're not easy to find.
1. The Fog Giant South of the Graveyard has a currently unused NPC in its mouth. It will tell the player that nothing is there... yet... and that it's lonely.
2. A duplicate of the player, called "your future self", is in a corner of one of the Abandoned House's rooms. Drinking it gives lots of juice.
3. Destroying all the graves in the Graveyard puts a Skull on the player's head.
4. Signy appears 3 times: in the Graveyard, on the head of the Fog Giant next to the House, and in a hard-to-access portion of the Cave. All 3 give different dialogue, and the one in the cave has a Pineapple.