Diglett's Cave and Meowth's House
Capture three Pokemon in the same location without changing location in between captures or getting a "Game Over." Wait for a new hole marked "Go to Bonus" to appear on the play field, and shoot a ball into the hold to reach the bonus level. If you're playing the red machine, the bonus stage will be Diglett's Cave. The blue machine's bonus stage is Meowth's House.
Mewtwo
Clear the Digglett and Gengar stages if you're playing on the red machine, or the Meowth and Seel stages on the blue machine, and then catch three more Pokemon or unlock another slot. Once in Mew's secret stage, hit Mewtwo as much as possible until he stops sending new balls into the play field. You can then leave the stage uncleared and go back to it whenever you want -- and keep racking up points, or hit Mewtwo three more times to clear the stage.
Mew
Mew is a rare bonus character worth a lot of points if you can get him to appear. If you're playing the red table, beat the Digglett, Gengar and Metwo stages twice. On the blue table, beat the Meowth, Seel and Mewtwo stages twice. Go to Indigo Plateau and enter "Catch 'Em" mode, and there will be a small chance that Mew appears. If he's present, hit him with the ball as many times as you can.
Animating Pokemon
An Easter egg lets you animates any of the unevolved Pokemon in your Pokedex: Open the Pokedex, select any unevolved Pokemon, and press and hold "Start" to watch the chosen Pokemon move around.
Bonuses
While in evolution mode, make sure to select a Pokemon that has a star beside it to take it through the evolution process, and you'll earn a 10,000,000 bonus for your efforts.
Light up the letters "C," "A," "V" and "E" to unlock the Cave bonus, but don't enter the cave. Light up the same four letters again before finally entering the cave, and the bonus will be worth double.
Pokedex Stats
If you're curious about how many Pokemon you've collected, press and hold "Select" until a set of numbers appears in the upper-right corner of the screen. The first number indicates the number of Pokemon you've caught, and the second number indicates the number of Pokemon you've encountered.