The Pokemart on Two Island
When you first visit Two Island, the local Pokemart will be severely limited in the items for sale. As you complete various tasks, more items will become available.

Unlockable How to Unlock

Soda Pop Rescue Lostella
Ultra Ball Rescue Lostella
Lemonade Obtain the National Pokedex
MooMoo Milk Obtain the National Pokedex
Repeat Ball Complete the Network Machine
Lava Cookie Complete the Network Machine
Timer Ball Complete the Network Machine

DITTO BREEDING
You don't need to find one pokemon of each gender in order to breed them. You simply have to put your pokemon of choice and a ditto into the daycare center. After a short while, the old man will tell you that they found an egg, and when it hatches, it will be whichever pokemon you matched up with Ditto

DETERMINING POKEMON FEELING TOWADS TO YOU
When you use an item on one of your Pokemon outside of a battle, such as a Potion, the Pokemon will either move closer or away from you in the short intermission sequence. How far the Pokemon moves depends on how much it likes you. If it does not move at all, it feels neutral.

FREE COINS
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.

FREE NEST BALLS
There's a woman that lives along the Water Path that's looking for a Heracross. If you catch one and bring it to her, she'll measure it and give you a Nest Ball as a reward. Each time you bring her a taller Heracross, she'll give you another Nest Ball.