UNLOCK TRAING WITH RUBY, SAPPHIRE AND COLOSIUM
At the beginning of LeafGreen/FireRed, you can only trade between the two respective versions. However, if you complete a certain task in the game, you can trade to Pokemon Colosseum on the Gamecube, and Pokemon Ruby/Sapphire, the two other Pokemon games on the GBA.
HOW TO UNLOCK
Defeat the Elite Four, then retrive both the Ruby from Mt. Ember and the Sapphire from Five Island Rocket Warehouse, then give them both to Celio
LORELIE DOLL COLECTION
For each 25 times you enter the Hall of Fame, she'll add a new doll to the collection in her house on Four Island. She'll stop after adding a total of eight dolls.
In the Gamecorner in Celadon City, go talk to the man at the far right Slots. He will give you 20 free coins. Talk to the man on the slots beside him. He will give you 20 free coins, also. Go to the left side of the Gamecorner. Talk to the guy that looks like a fisherman, and he will give you 10 free coins.
LOOSE CHANGE IN GAME CORNER
Apparently, the folks in the Game Corner don't hold on to their coins very well!
By using the Itemfinder or looking by yourself, you'll find that many spots in the Game Corner have some number of coins lying on them. If you find them all, you'll find 200+ coins!
SAVING TIME IN SAFARY ZONE
When you are in the Safari Zone, it counts the number of steps you take to determine the amount of time you have left. Well, if you want to catch a rare Pokémon, but don't want to run out of time, enter the grass, then lightly tap the directional pad. You will change the way you are facing, but it will not count as a step. You can have near-infinite time in the Safari Zone doing this. It is helpful when some Pokémon, such as Chansey, appear in only one section of the Zone.
Every time you encounter a pokemon you have a 1/8192 chance of the pokemon being a different color, this is called a shiny pokemon
NATIONAL POKEDEX (JP VERSION)
Once you have beaten the Elite 4 and have more then 60 pokemon in the pokedex, go to Pallet Town and see the Professor. He gives you and Gary the National Dex, which allows you to view the entries of all 386 pokemon.