Unlock the following cheats by entering the corresponding Wonder Mail codes:

CHEAT FUNCTION - CHEAT CODE
Apple Woods 7F (A); Reward: Gold Ribbon, Big Apple - =42#H TF&QRM8 =0NJ+R4MHW R-TN2#0 8C@2#
Apple Woods 8F (B); Reward: Recruit Combee [F] - =R-S0 R-5M-K% C&P6-K#503 S80&&H& RW4J6
Beach Cave B2F (D); Reward: 150 + Black Gummi - w+S06 @PHR0PW 26X-6#10-H Q%FPNW2 4Y1#0
Blizzard Island 13F; Reward: Lunar Ribbon - CNJJN9-PSJNXX-NFH
N2XT@XS2501K565KP
Drenched Bluff B3F (C); Reward: 200, Reviver Seed - R7&20 =M34M3M 6-=YNW@543 W--N%KT P&4PQ
Mount Avalanche 17F; Reward: Sun Ribbon + ? - 4C-NRH@R4+J6+@F0
7R&2MXFF739S8&##J
Mystifying Forest 7F, Explore with Mismagius; Reward: 550P + X-Ray Specs - C=5-=J31T=0&67Q5QQCN4SRFQNYPH=FN1-
Sky Stairway 40F; Challenge Mewtwo and recruit him - CK9XSC-PQMSK4F48H00XTN-=46719T&WK1
Southern Islands B13F; Challenge Raikou and recruit him - WS7994C2FTR2SN241%@HKY#+#649JFY=7C
Zero Isle South B74F; Reward: X-Ray Specs + ?  4CH58255XQ#QJ3WYH0F0P@75PWMC&NK9T

Unlock the following dungeons by performing the corresponding tasks after completing Chapter 10. After completing a bonus dungeon, a treasure chest will appear on the last floor:

DUNGEON - HOW TO UNLOCK
Bottomless Sea - Get the "Secret" rank, and obtain the job from Spinda's Cafe.
Destiny Tower - Beat the game, and make drinks at Spinda's Juice Bar.
Giant Volcano - Get the "Secret" rank, and obtain the job from Spinda's Cafe.
Happy Outlook - Beat the game
, and make drinks at Spinda's Juice Bar.
Inferno Cave - Get the "Guildmaster" rank.
Labyrinth Cave - Beat the game, and find a job with ??? for the Place.
Lake Afar - Make 100 Recycles.
Landslide Cave - Make 3 Recycles.
Lost Wilderness - Beat the game, and make drinks at Spinda's Juice Bar.
Lush Prairie - Make drinks at Spinda's Juice Bar.
Midnight Forest - Beat the game, and find a job with ??? for the Place.
Mt. Avalanche - Get the "Secret" rank, and obtain the job from Spinda's Cafe.
Mt. Mistral - Beat the game, and make drinks at Spinda's Juice Bar.
Mystery Jungle - Get the "Secret" rank, and obtain the job from Spinda's Cafe.
Oblivion Forest - Get the "Master 1 Star" rank.
Oran Forest - Make 60 Recycles.
Serenity River - Make drinks at Spinda's Juice Bar.
Shimmer Desert - Get the "Secret" rank, and obtain the job from Spinda's Cafe.
Shimmer Hill - Beat the game, and find a job with ??? for the Place.
Sky Stairway - Get the "Secret" rank, and obtain the job from Spinda's Cafe.
Southern Islands - Get the "Master 3 Star" rank.
Star Cave - Beat Special Episode 1 with the "Secret" rank, then get Jirachi's Challenge Letter from Spinda's Cafe.
Tiny Meadow - Make 25 Recycles.
Treacherous Waters - Get the "Master 2 Star" rank.
World Abyss - Get the "Secret" rank, and obtain the job from Spinda's Cafe.
Zero Island Center - Graduate from the Guild, and make 150 Recycles.

If your partner or you is a "Treecko", teach it Dig. You also need to get a Violent Seed and an X-Eye Seed or two.When you reach Dialga or Palkia, before they can use any moves, simply hit them with an X-Eye Seed. If you miss, they will not attack and will just pick up the item. Once they are under the mighty doom of the X-Eye Seed, give the Violent Seed to whoever is the Treecko and then use Dig when Dialga or Palkia are close. The attack does 500 or so damage for me, and one blow KO'ed them. Even better, if you or your partner is a Pikachu with the Treecko, you would then teach it Agility, which will give you up to four attacks in one turn.