To get the other Lati (Latios or Latias), Mix Records with a Pokemon Ruby/Sapphire game that used the Eon Ticket. You can then get to a hidden island via the SS Tidal where you can find the Lati you didn't choose.
On Route 113, to the right of Fallobour Town, is house in which a man will make you items if you bring him ash. To collect ash, walk through the ash covered grass. Then bring it to the glass item maker.
To do this you need an Everstone. To get one, search in Granite Cave for a lone rock on top of a plateau. Once you get an Everstone, let the a Ditto or a female Pokemon that you are breeding hold on to it. Once you receive and hatch the eggs, the offspring will have a 50% chance of inheriting the Nature from the Pokemon holding the Everstone.
To do this glitch, you must first execute the Pomeg Berry Glitch, which is done by using a Pomeg Berry on a Pokemon with 1 or 2 HP so that its HP loops to 65,535 (displayed as ?35 in-game). Do so, however, with only one Pokemon and an Egg in the party, with the Egg in the top-leftmost slot of the party in the Pokemon menu. After executing the glitch, you will now be able to enter battles and use the Pokemon Egg as if it were a normal Pokemon. It will function as a normal Pokemon - you can view the stats it will have upon hatching, it can level-up by beating Pokemon, it can learn new moves, it can even evolve. This means that you can alter the Pokemon inside the Egg in otherwise-illegitimate ways. For example. Pikachu cannot normally know Thunder when hatching from an Egg at Level 5; through this glitch, Pikachu can. Similarly, you can hatch Eggs with fully-evolved Pokemon such as Typhlosion, Meganium, and so on, inside.
If you have a pokemon with the Magnet Pull ability in front of your party, it increases the chances of finding wild steel-type Pokemon. This is very useful for finding rare steel types, such as Mawile.
Mirage Island is an invisble Island located on Route 130. Most of the time it will be invisible but on rare cases, the island will be visible. To find out if the island is there or not, go to Pacifidlog Town and talk to the old man in the bottom right house looking out the window. He will tell you wheter the island is there or not. The cause is unknown so a bit of luck is needed to find it. On the island you will find the rare Leichi Berry that powers up your Pokemon's attack in battle when it gets weak. In the grass, you will only find Wild Wynauts.
To unlock mystery gift, go to a pokemon mart and press A on the piece of paper on the counter. Then insert the words LINK TOGETHER WITH ALL. Press select to easily find the words. After this the store lady will talk to you and tell you that mystery gift is now unlocked.
For this glitch to work properly, you must have a Pomeg Berry and a Pokemon who has not had their HP lowered as much as possible already through Pomeg Berries. (If the latter is the case, the use of HP Ups on the Pokemon will suffice.) Get the Pokemon to a severely low level of HP (1 or 2 HP). Once you do so, in the field, use the Pomeg Berry on the Pokemon. This will in turn cause the data in their HP to cycle back to the top of the counter, giving the affected Pokemon 65,535 HP - over 90 times the otherwise possible maximum in the game.
Talk to Scott in his house in the Battle Frontier, he'll give you certain decorations/berries depending on your accomplishments in the Battle Frontier.