After completing any new stage for the first time, you'll have the option of replaying those stages via the "Replay Machine", which is located on the far right of the Garage and accessible via a small pc. Upon any completing any stage again, you'll be ranked via stars as with the previous titles. Each stage can give between 1 to 5 stars depending on your rank (totaling to 105 stars once every stage is completed again).

Unlockable	How to unlock
Concept Art (Grocks/Bosses)	30 Stars.
Concept Art (Landscapes, Stages, town, misc)	50 Stars.
Concept Art (NPCs, playable)	10 Stars.
Cut scene viewer	5 Stars.
Developer Message	105 Stars.
Sound Test	100 Stars.