From Bulbapedia, the community-driven Pokémon encyclopedia.
This is the Bulbapedia walkthrough for Pokémon HeartGold and SoulSilver Versions. This walkthrough follows the remade Nintendo DS versions, not Pokémon Gold and Silver.
The guide for those can be found here.
Gathering the Forgotten Items
Even though the game is done, there are still many items that you may not have obtained in the earlier areas without the right HM move. Once you've obtained the eight moves, you can track down the following items.
• Surf to the west and speak to the guy on the island to get the Mystic Water, which boosts Water-type attacks by 20%.
• Surf southeast across the pond to reach a hidden Nugget.
• Smash the rock near Earl's Pokemon Academy to reach a path with a hidden Hyper Potion at its end.
• Surf east from the bridges leading to Sprout Tower to find a secluded area with a Rare Candy.
• Cut down the tree to reach the guy on the ledge, who gives you TM05 (Roar).
• Smash the rock south of the Poké Center to reach a Shell Bell.
• Swim out into the water west of the Daycare building, and surf south along the shore. A Nugget is on the other side of the fence from Picnicker Gina, and a Rare Candy is hidden on the ground nearby.
• Continue south to find the three Ace Trainer sisters. The first two fight in a tag battle, and the third will reward you with a Power Herb for defeating them.
• Surf across the pond to reach a Grn Apricorn tree. There is also a hidden Nugget on the ground to the north of the tree.
• Walk through the break in the fence and head east. Use Rock Climb to scale the cliff and find a Shiny Stone, which can evolve Togetic into Togekiss and Roselia into Roserade.
• South of MooMoo Farm is a scalable wall to the east. Follow the trail back to Route 38 to find a Lax Incense. There is a hidden HP Up nearby, as well.
• Climb down rock formation next to the Lighthouse and examine the rock there to find a Rare Candy. If you haven't already, you can also surf around the Lighthouse from the docks to reach TM57 (Charge Beam).
Cliff Edge Gate
• Scale the side of the rock wall and surf to the northeast corner of the pool to find a Big Pearl hidden on the rock.
• South of Dark Cave's entrance is a rock formation that leads to an empty plateau. A PP Max, the equivalent of using three PP Up at once, is hidden here.
• If you haven't yet, visit the Celadon Mansion after winning all sixteen badges. Talk to the Game Freak staff to receive the GB Sounds, which lets you play the original music from Generation II.
• Diglett's Cave has several scalable rock walls. A PP Max can be reached just south of the ladder. Farther along are adjacent rock formations; the left leads to a hidden Calcium while crossing the bridges to the right leads to a Rock Incense, which enables Sudowoodo to lay Bonsly eggs.
• Climb the rocks scattered atop the volcano to find a Star Piece and an Iron, both hidden. A Magmarizer, which lets Magmar evolve to Magmortar when traded, is lying in plain sight on the ground. A hidden Rare Candy is in the northwest corner of the island, as well.