Alternative way to enter the gallery - dig up the key. You can find a shovel in the staff room (2 times to the right of the room, the door on the right) and then dig in the spot on the screenshot (on the beach, go to the left until you end up at the empty beach).
I also found some of the scenes. Triggering each scenes proceedes to the next day.
The scenes that I found are:
1) The blue haired maids give you a hint that there's a ghost in a "place that hosts lots of people". Go to the conference room (go to the left exit in the hall after you leave your room, first door after the toilets) and stay there (don't move), in like 10 seconds a message box appears. Go down to the table and inspect behind it - the scene will trigger.
2) Rigth outside the quest gallery on the street (go straight to the right when leaving the room, keep going until you're in the plant hallway, then go down and from there down again) in the trashbin at the lower right side of the screen there is an ice cream pass. Give it to the ice cream girl (rightmost side of the beach) to play the basketball minigame. Winning the minigame triggers the scene.
The minigame is kinda bullshit so I have no idea how to win legit, I cheated to lock the amount of shots that I have and just made sure I landed 10 shots before I left the minigame.
3) In your room - talk to the girl on the bed - she will give you the markers. Now exit the room, exit the hallway through the right side, go past the reception to the right, enter the room on the right - you will be in the staff room. Use the markers on the board, choose pink color of marker and write SOFT RUB. Exit the staff room, go right (you will enter the plant hallway), then go up and straight into the first door. Talk to the pink haired girl and you will make the game freeze. I think it's supposed to trigger a scene - but it doesnt, the game hangs.
The way to check if you've done it correctly is to go to the spa (a door to the left of the massage room), go to the right wall and inspect it - there the girl will say something. If she talks about "wrong color" - you've made a mistake, otherwise she will say that she knows exactly what to do.
4) The last one seems to involve the one-handed girl from the pool. She asks to find the ring, but I have no idea where to search, and tbh trying to figure out the previous three was NOT worth it.
The code to the shack is 2809, there you can find a metal detector that will help find the key on the beach.
The scenes are cute, but not worth the effort IMO - they are without any penetration.
BTW I checked the game files - there's only Megan masturbation scene.