I apologize if you found the ending unfulfilling, but the first ending is in fact the canon ending. All post-game content is mostly for people seeking more challenge. There is another ending, but...
But to answer your question, go to the Princess Zone. There are a few doors hidden behind pillars. You can use the Blue Face there. If you want another hint, there are things hidden in the Mind Mines aside from just Fortune Tickets.
Thanks for playing and sorry for any frustration!