Ploughing through this a bit more today. Got myself up to 50 stars, meaning I could reach the third floor of the castle (and access Tock Tock Clock and Rainbow Ride). The next target is 70 stars, which, if I remember correctly, is all I need to complete the game.
Went into Rainbow Ride and got one star, as well as open up the cannon there. Didn’t try for any more after that though. Don’t I have Zelda to play or something?