In Hoshi Saga 2, you are presented with a series of mini-games and the object of each is to find a hidden star. How the star is hidden and how you find it varies with each game. It’s kind of Wario Ware-ish in that the fun comes from figuring out what you’re supposed to be doing (minus the time pressure).
The levels are presented in a grid, and at the end of each row and column is a hidden mini-game that can only be accessed once every other game in the row/column is beaten. Beating all the hidden games unlocks the credits.
The games are all very clever, and beautifully presented. Quite lovely!
And be sure to try Hoshi Saga 1, too.
(Source: Sensible Erection [NSFWish])