Get past the initial terrain; ahead of you, the building with the blue key prominently displayed. Go to the right, drop down in the lava trench and go inside. On the right, a small platform with a marble wall behind it. One of the panels are a concealed door. Secret inside. Go to the end of the secret room and take the lift up.

From #1, jump down and go to the other side of the building (avoid the lava for now!) - walk into the cliff face at the big tree. There is a walk-through wall, and goodies inside.

From #2... jump in the nearest lava and go inside. Another platform like in #1, another concealed door... Carry on along the left side of the corridor, all the way to the end. At the bend just before the end, the secret triggers. There is a door, leading to a lava pit and a goodie. I don't know if it's worth it...