I completed this game a day ago. Found every secret. Secret stages were mad fun with more difficulty. Prooving Nintendo still can make those kind of stages.
I really had only problem with one thing. That was a secret coinblock revealing a tarzan whip to the clouds. Seriously, that thing was well hidden knowing the stage. For the rest it is all obvious.
Still great fun this game and hopefully next time even a bigger step in difficulty. I also love koopalings they own Bowser jr for sure. Oh and the last battle rocked to.