Difficulty Changes- If you beat the game, you unlock "Hard" difficulty.
The bosses are harder, and so are the enemies. A LOT harder. Upon
beating hard mode, you unlock "Easy" mode. Money seems to come in
faster, enemies and bosses are very weak and the game is generally a
piece of cake.

New Game Prizes- If you beat hard mode in under eight hours, you can
start a new game with jet skates and the buster max. Also, when you
start Easy mode you will always have the buster max. As for as I know,
those are the only two prizes you can get. I beat the game in 1 hr, 40
min and those were all I got. Yes, I did say 1hr, 40 minutes ;).

Burger Shop- If you solve most ofthe peoples' problems, during the part of the ending where you can walk around, there is a burger shop Downtown! (weird, huh?) As for what it is for, I have no clue.

There may still be more...