Turquoise slimes might be a future thing
Waterfall cave only has the coin. There IS a secret boss enemy in the other end of the cave. You can get there from the Light Forest map where the statue is. You need to find a key first to open the gate, which you can find in Logtown.
Abandoned house isn't much more than the note. The note will be more important later on. Patreon version currently has 3 notes.