From your starting position, dive all the way down in the lake to find a secret rocket launcher.
At the lake, head to one of its corners with an island to discover a secret armor.
From #2, walk towards the phone booth to register the secret.
Walk to the left side of the temple with the Sniper, and near its end, blast a darker colored section on the wall to reveal a secret "under construction" area.
Opposite from #4, walk towards the mountainside to discover an item.
When you get out of the temple and are in a valley, turn around and look up. You'll see a platform with an Invulnerability power-up. Blast it with a rocket and collect the power-up (although the secret will not register).
Go to the upper right corner of the valley to discover a secret armor and spawn some enemies.
Opposite of #7, walk to the lower left corner of the valley to discover a "Crollywood movie studio".
At the middle mountain with the large pyramid, walk to the left and enter the house there to find some secret ammo.
Opposite of #9, walk behind the house on the right to discover a health pack.