Hi, thx a bunch for your comment :)
* spoilers below *
I am going to SPOIL some things that you can achieve: there is a fixed amount of seeds in each layer. Growing flowers from these seeds makes markers in the sky light up, so you could try getting all of the seeds and reading all the notes. Another goal to strive for is finding the glasses which allow you to switch into a screenshot mode. To get the chest at the bottom you need another item (found higher up).
My personal definition of 100% is all seeds, notes + glasses + chest. But tbh I think it's unrealistic to expect getting everything, you've reached your personal 100% when you feel no need to play further (;
Oh, and a basic thing that players sometimes miss is the ability to create a crossing in a spot of choice after every grown seed (aka using your charge).