I wish you could get upgrade stars for beating it the first time (whether it be easy or normal) then you could get some more for beating it on hard for the first time
Another secret is in Level 2, where you can push the first blue platform (there are two blocking your path), and you can fall through the floor. Note that you need to run and jump to get over the spring
In Obsidian Yards, you need an Alchemist to turn golden apples into regular apples. Does that help?