I can't find the guitar and the baseball cap!

[JFK_360NoScope]

There's a painting that is obviously telling you to set up the shelf with certain items. The only two items that I can't find are the Guitar and the baseball cap! where are they!?

[Buddy]

You need to go though the doors marked moon (only opens at night) and sun (only opens at mid-day) and to your right is a door, you need to drain the room then go in

[Padelboot]

I will try to explain it as best as I can, though I suggest watching a video on YouTube if I am not clear enough. Obviously I will go a bit into detail, so I may help solving other riddles as well. I marked it as a spoiler in case people want to figure it out on their own and just want to know where it is.

The baseball cap is in the same room. When you are in the corridor and about to enter the room look to the ground to your left. There should normally be the cap.

The guitar is a bit trickier. It is behind the night, door, the sun door and behind the Spongebob style door, which however leads to a flooded room.

So, you will first need to get to the door with the moon above it, which is best accessed by the rails. You need to enter that room in night, otherwise the door won't open. So block the night door with a chair and it will be open during broad day light. Because the next door will only open at broad daylight (this is the same room you find the phonograph with the positive music!).
Also make sure you let the water out: Find the room where the stationary "weapons" are shooting apples and climb out of the window. Look to the left and go to the pipe. Use E multiple time on the red handle until the water comes out.

With this done you can walk through the once flooded room and...voila! A small room with a cake and the guitar! And also a way to quickly access the child room where you have to place the guitar.

I hope I could help you out a bit. :)

Edit: Damn, a bit too late. Oh well.