This is a wholesome, fun and family-friendly puzzle game. I love the story and how you tell it with pictures at the start - also, I'm a big fan of the hand-drawn art style. The music and sound are excellent. The bunny's voice adds to the charm of the character.
The game controls felt intuitive. I liked how the first few levels were simple, and the more complex controls / moving the cube around were introduced later to ease the learning curve. I appreciated having the option to restart the level and that the camera goes back to the player when returning to 2D from 3D.
In terms of difficulty, I found some very simple to solve while others took me a long time to figure out. To make the game enjoyable for more people, you may consider adding optional hints/tips.
I noticed some of the levels had both a regular egg and a golden one. While I decided to stick to just the regular egg, having them there adds a bit of curiosity and extra challenge. I love how you give players the choice of which one they want to try to get - and still let them continue on with the regular egg.
Something I felt was missing is a strong sense of progression. After solving many levels, I wondered, how many more levels are there? To better set players' expectations, you may want to consider ways of communicating this. Such as adding additional information to the start of the level when you show the title, "Ruby's Attic: Up the Stairs (level 1 / 15)".
Even though it's mentioned that you can hit ESC to go to the menu, for my first play, by the time I watched the story and played the first handful of puzzles, I didn't remember what was written on the "how to play" screen or itch page - so didn't realize that there was the menu (and level select screen), until going back to the read the itch page to see if it said how many levels.
Since the title screen only shows the level selection option if there's saved data, other first-time players might miss this as I did. I would have liked more guidance around accessing the menu and how many levels to expect. For example, perhaps the first thing shown after the story is the level selection screen, so players learn that it's there and see up front where they are in the grand scheme.
Suppose you feel that right after the story would interrupt the flow of things too much. In that case, another idea could be to show the level selection screen upon solving the first puzzle. When the player selects the next level, there's an opportunity to tell the player about the menu screen (similar to how you currently tell the player how to press space to go into the 3D mode).
Overall very fun, cute and polished game! Well done :-)