If you haven't already, I highly recommend ayeT Studio's Magic The Gathering Arena.

To complete this offer, you need to get to level 25 and you get 675 SB upon completion. I know it might not be the best SB to time ratio, but MTG Arena is a damn good game and for once on SwagBucks, you'll actually have fun doing something. However, if you do want to speed through it, there are plenty of pre-designed decks and tons of guides on how to get more wins and be a better player made by people far more skilled than me.