While doing an RA for Silent Forest 3 in Sonic Lost World on PC I noticed one thing; In the second part that you get to by using bounce on purple ice there are ice crystals that give you rings. When you collect all rings from them, go backwards and then forward some of them will reappear. Said crystals will have rings again thus making it possible to infinitely farm rings on them (or at least until the time limit's out). This was tested on PC version, not sure about the Wii U one.
EDIT: In fact, going out of the area with said crystals and then going back makes them respawn.