Plus, the reward for winning each of these games the first time is a Hero Coin (one step closer to beating the game 100%) and you won't get anything for winning again

Swanky has his original challenges that take coins to play for lives and cannot be replayed, but now he also challenges you to pay 10 coins every time you want to re-fight a boss, with a promised prize of 6 extra lives if you win--ultimately, it's a quick but very challenging and expensive way to win lives that is far superior to the overly easy Animal Bonuses of DKC.

In addition to the games bonus coins, you'll also find "hero" coins.