Grandstander

Grandstander (also known as Performance in Kingdom Hearts II Final Mix) is a job/minigame that can be played as Roxas or Sora in Kingdom Hearts II, and by Roxas again in Kingdom Hearts: 358/2 Days. Similar to a Rare Truffle, the point of the game is to keep the ball in the air by attacking it for as long as you can.

Sora's Reward
To unlock the secret ending, you need to complete Jiminy's Journal. Part of finishing Twilight Town's entry involves playing this mini-game. The Jiminy Objective is to complete with 100 or more points.

Roxas's Reward
If this mini-game is completed with a certain number of juggles, he will earn various amounts of munny.

Kingdom Hearts: 358/2 Days
Mission 37: Vacation has Roxas take part in Grandstander training. There is no actual reward, but doing 50 hits or more will grant access to Mission 49 later in the game.

Strategy
Each hit knocks the balloon up slightly and each combo finisher sends it flying. Lock on and try to hit the balloon toward a corner. If you swing too early, you will likely lose your combo.

Advanced Tactics
Equip a long weapon prior to doing this task. This increases your range, thus minimizing your chances of missing. "Combo Plus" abilities are useful, but do not equip a finisher technique. If the technique takes too long to perform, a ball that is hit far away may hit the ground before you can approach it.

Another good tactic is to equip Fatal Crest or simply the Berserk Charge ability, lock on, then knock the ball into the air. Knock it high, cast Cure then jump up and continually slap the ball. Eventually you will reach the top of the map and be in the air beating the ball and adding hits, this will continue until your MP fully recharges. A good round of this will can add over 100 hits.