Get Powerful Attacks on Level 10 Pokemon
After you win the match against your rival,the Move Tutor will appear just outside of the Goldenrod City slot machines.He can teach one of your Pokemon Flamethrower,Ice Beam,or Thunderbolt.He can even teach level 10 Pokemon!
Get rare items
In the Ruins of Alph sliding panel room,there are other secret rooms that you can get access to by performing a certain task.Those secret rooms contain 4 rare items.
Equip a Pokemon with the item you want to duplicate.Deposit the Pokemon in a box.Now,change to another box.When it says "Saving" immediately turn off the power.Your Pokemon and the item will be duplicated.
Get Lots of Rare Candies
Go to the Radio Tower in Goldenrod City.Talk to Buena to get a point.Save your game and turn it off.Turn it back on and change the time to the next day.Talk to Buena again to get another point and trade it in for a Rare Candy.Do this as many times as you want until you have enough.
Get to the First Secret Room
Go to the first puzzle room.Read the writing that's on the wall.It will say "escape".That's the clue.Use an Escape Rope.You should be outside now.Go back into the room.A door will have opened.It is the first secret room.
Second Secret Room
Go to the southern part of the ruins.Surf across to get to the second puzzle room.Read the clue.It says "light".Use flash by there.A doorway will appear.
Third Secret Room
For this, get to the third puzzle room with a water stone in your pack. The passage will open.
Forth Secret Room
This is the final room. You'll have to get all of the ledgendary dogs to get to the room. After you get them, catch Ho-oh. That's your ticket to get in the last secret room.