See the level for details on reaching the Bonus Area. In terms of the rewards you get, Swipe Swiftly's Wicked Ride is the best level, as you get five Hearts and 46 crystals, although to get to the Bonus Area, you must play for a couple of minutes. It would be better to go to Fight Night With Flibby or I Snow Him So Well which also contain five Hearts, although no crystals but can be reached earlier in the level.