The Sinnoh region is full of hidden items and secret areas waiting to be discovered. Always use your Dowsing Machine Pokétch app (obtained on Route 207) to find invisible items on the ground. Some notable secret areas include:
- Wayward Cave (Hidden Entrance): Below the Cycling Road (Route 206), there's a hidden entrance to a deeper part of Wayward Cave that requires Strength. This is where you can find Gible.
- Old Chateau: Located in Eterna Forest, this haunted mansion is home to Ghost-type Pokémon like Gastly and can be explored at night for rare items and encounters.
- Stark Mountain: A post-game area accessible after becoming Champion. It's crucial for the Heatran quest.
- Fullmoon Island / Newmoon Island / Flower Paradise: These are event-exclusive islands for Cresselia, Darkrai, and Shaymin, respectively. You need specific Key Items (Lunar Wing, Member Card, Oak's Letter) to access them, which were distributed via Wi-Fi events.
- Spear Pillar (Post-Game): After defeating the Elite Four, you can return to Spear Pillar to find the Adamant Orb (for Dialga) and Lustrous Orb (for Palkia).
Thorough exploration and the use of HMs will reveal many of Sinnoh's hidden treasures.