The Reward items can currently only be collected by performing actions in the game.
You can get the card rewards by collecting all 60 in-game cards from the freeplay, 60 in-game cards from the lava-mode, both in the school and in wholesale. The 3 reward stickers can be obtained by completing the pogo stick challenges. By collecting the 3 challenges in school, you get one of those stickers, same for wholesome. I assume you get the third sticker by completing the pogo stick challenge in the mastery-level, although I haven't finished it so I can't confirm it.