In order to teach your Pikachu SURF which is a technique it would not normally learn, you need to first unlock the Round 2 mode. Now do the following:
Enter the Round 2 Prime Cup's Master Ball division. In order to do this you have to beat the other three divisions first.
Assemble a team that contains a Pikachu that does not yet know SURF. You must use a Pikachu from the Game Boy game, which means you cannot unlock Surfing Pikachu using a rental Pikachu.
Do not register your team. Play the game directly from your Game Boy cartridge.
You now have to beat the Prime Cup's Master Ball division using a three-Pokémon team that always contains Pikachu. Pikachu must be in every one of the eight battles, and you are not allowed to use any continues. If you do, you won't get Surfing Pikachu.
Once you have fulfilled all the above conditions and beaten the cup, you are shown an award screen with Pikachu standing on a surfboard. From here on, Pikachu can use the SURF technique both in battle and during gameplay in your Game Boy versions of Pokémon. If you own Pokémon Yellow, you can now also access the hidden Surfing Pikachu mini-game. Simply go to the small hut on the northern beach of Route 19.
To unlock a hidden 6-on-1 battle with Mewtwo, beat the Gym Leader Castle and get trophies in all 10 tournaments in the Stadium Mode. The map screen will now turn to a night scene and a bonus tournament will open up in which you battle against the psychic super-Pokémon, Mewtwo (#150). Once you beat Mewtwo, you unlock a new game mode, Round 2 mode. This is harder than Round 1 mode. You can toggle between the two at the start up screen by pressing C-Right.
Alternate Pokemon Colors
It is well known that there are different color pokemon in Stadium. The secret to the color variations lies in the names you give your Pokémon as well as your trainer ID number (assiged to you in Pokemon Blue, Red or Yellow, whenever you start a new game). While there are a few secret names that change only certain Pokemon's colors, there are a few naming tricks that result in different color variations. For example:
Use only lower case letters (ie: poliwhirl)
Reverse the syllables (ie: DRILLBEE)
Abridge the names (ie: Pika)
Each time you beat the Gym Leader Castle, which includes the Elite Four and your Rival, you will receive one of eight semi-rare to rare Pokémon for your Game Boy game. These Pokémon are awarded at random, but the good thing is that once you have beaten the Castle, you only need to defeat the Final Four and your Rival again if you want to win more Pokémon. The Pokémon you can win are Bulbasaur, Charmander, Squirtle, Hitmonlee, Hitmonchan, Eevee, Omanyte, and Kabuto.
Trade Without Cable
While using the sophisticated PC in Prof. Oak's lab, you can load Pokémon from one version of Gameboy Pokémon, save it to the N64 gamepak, and transfer it back to another Gameboy version.
You will get the Pokédex data from the Pokémon, and you will be able to use them in gameplay. This trick can also be done with entire boxes of Pokémon! The only problem is that you can't trade items.
GB Speed Upgrades
There are two secret speed upgrades to the GB Tower that let you play Pokémon Red, Blue and Yellow at much higher speeds:
In order to get the first one, Doduo Mode, beat the Poké Cup. Once you have won all the Poké Cup tournaments, the GB Tower changes into Doduo GB Tower and a Doduo sticker appears on the Game Boy. The Doduo upgrade doubles the speed of the game play when playing the Game Boy game on your N64.
In order to get the second upgrade, Dodrio Mode, change the speed settings to Hyper Speed and play the Game Boy games. Then, you must beat the Prime Cup after already beating the Poké Cup.
You can switch between the different GB Tower speed settings during gameplay by pressing C Right.
Go to the Kids Club. Play a one player game against the computer in "Who's the Best." Set the difficulty to hard and put the number of wins to 5. If you can beat the computer 5 times in a row without losing a single game, a new difficulty mode will be unlocked called "hyper-mode." This makes the CPU much faster and therefore much harder.
Catch a Missingno. in the RBY versions of Pokémon. Upload them into your 64 and look at his stats. Missingno. will have a picture, although he will have no Pokédex entry. You can see a picture of Missingno. HERE. There is no cause for alarm, because Missingno. doesn't seem to affect the Stadium Gamepak, but there are no promises made here.
Once you have beaten both the Round 1 mode, including Mewtwo, and the Round 2 Mode, including Mewtwo, you are given a set of bonus stickers that you can access in the Gallery Mode. When you're on the Gallery main screen, press and hold L and R and move the Cursor to Print. If you keep the buttons pressed, the Print option will now change to Bonus. You will now have access to a set of 16 bonus "wallpaper" stickers from all the different modes that you can print out at "Snap" Stations at your local Blockbuster Video.
Get Amnesia Psyduck
In order to get Amnesia Psyduck, you must register all 151 Pokemon in the Hall of Fame. After doing so, you will be awarded with the hard to find Amnesia Psyduck.
If you use your Pikachu from Pokémon Yellow in Pokémon Stadium, it will utter its trademark "Pikachu!" phrases during battle. A rental Pikachu, or one uploaded from Pokémon Blue and Red, however, will remain silent.
Put Pokémon Yellow version in the Transfer Pack. Go into Gallery and upload Pikachu in the Snap version of the game. He will pose differently than if you had uploaded him from Red/Blue.
New Title Screen
Beat both the Gym Leader Castle and Stadium modes to change the title screen. The new title depicts Pikachu, Blastoise, Charizard, Venusaur and Mewtwo.
How To Get Different Picture Arenas
As you continue through your game, you will battle in different arenas and stadiums. After you have battled in a new one, you will be able to photograph your Pokémon in that arena using the Gallery Mode. While snapping photos, press L and a list will come up of all the different arenas. As you battle in new places, different stages will open up.
Different Pokémon in "Battle Now"
When you enter the R2 mode of "Battle Now," you will notice that the Pokémon that they give you are different. They are stronger and many are the evolved forms of the Pokémon that can be used in the R1 mode.
When you are on Clefairy says, look at the arrows and press on them. After the Clefairy start singing, press start on your controller and wait about 10 seconds. Press start again, and when the Clefairy move their arms, there won't be any music!
While doing an attack press A and about 90% of the time you will do more damage than usual.