Bikini Girl
If you complete the first 10 rounds in under 20 seconds each and get a perfect score in each bonus round, you should have over 240,000 points and a window will appear in the lower left corner of the screen with a girl in a bikini.
Level Select
Start a new game. While the game is loading, hold Up + A + C + Start. Release the buttons when a "Round 1" screen appears, then use Up or Down to choose your starting round.