How many rewards you get and how good of rewards depends on how well you do at the minigame. When it shows your score after the game, you can see the first box go bronze, then silver, then gold, then the same with the second box, and so forth. Where it stops depends on how well you did.
The first box is mana (or potion refills if your mana is full), the second is a bit of gold, and the third and fourth are items or treasure cards. The items and treasure cards are invariably very low level, no matter how well you do. I've put a number of scores on the top 10 of Choo-Choo Zoo, most of which have since been knocked off the list, and most of which resulted at the time in getting the fourth box to silver. I still have the #2 score as of not very long ago, and that score got the fourth box to gold, but the items and treasure cards in the third and fourth boxes were still very low level. I don't think I've ever gotten any items other than armor (hats, robes, or boots).
Quizzical, that was nice info but not really what Jonnyboy asked.
I've never gotten past the second reward myself but have always found it to be some type of clothing and different for the different games. For those in the high score areas, what would you suggest for a game to try?