Right after crossing the first ramp you will see a pile of boulders with a skeleton hanging from them. Destroy the outer layer to unveil a mithril boulder, change to Berserker and blow it up. now just pick up the Minikit.
Treasure: Trophy Shield
Right before the first lake of fire; destroy the nearby boulders and build a platform from its remains. Afterwards fish up the treasure from the lave.
Needed: rope, climbing, acrobatics
Beside the first "elevator", head onto the rock ledge using the rope, change to Gollum and climb up the wall. There, grab onto the rail and head left towards the river of lava.
In the upper left corner of the screen you should see a chest - your target.
The tricky part begins here. You need to jump onto the falling rock, change to Legolas and jump between the rocks fast enough to reach the Minikit in the upper-left corner of the location. You should make it after a couple attempts.
After stopping the river of lave, dig up another stone from the loose ground beneath you and throw it at the pillar on the right. Now just cross it and pick up the chest.
In the place where the rocks are falling down, as you're crossing the river of fire while holding onto the rail, you will see a cracked LEGO tile. Just destroy it with Gimli's axe to receive a Minikit.
Needed: bow, magic
After crossing the river of fire where you found Minikit #4, you will find a few colorful bricks above you. Shoot the target and use Gandalf's spells to build stairs which you can climb to the treasure.
Needed: Pippin's bucket
After obtaining Minikit #5, fill up the bucket in the nearby pond and extinguish the fire. Now enter the unveiled narrow tunnel and destroy the chest on the rock ledge.
Treasure: Palantir Hat
Needed: strength, spade
The rock with the orange handle is visible far on the right side.
From the place where you obtained Minikit #6 you need to head to the right. Jump over two precipice and you will reach a stone with an orange handle. Pull it with Shagrat and dig up the ground with the spade. Pick up the treasure.
Design: Mithril Climbing Boots
From the place where you found the treasure, head further to the right and north - climb up the rock ledges (start with the lower one, jump to the left to the one with a stream of steam and from there to the very top) towards the door to Mount Doom.
Beside the gate you will find a target for Legolas - shoot it with his bow and jump to the Design using the therefore created bar.
To the right of the Design you will find a small LEGO Morgul brick. Destroy it and use the platform to get onto the rock ledge to pick up the chest.
After blocking the lava flow, right before the first fight with Gollum (to the south of the door to Mount Doom); just enter the river-bed and approach the camera. Jump down the grab the Minikit.
Inside Mount Doom; jump onto the rail behind you and grab the chest.
Treasure: Magna Rock
Needed: Mithril Boxing Gloves or Calcium Armor
Inside Mount Doom; when Gollum throws you down for the second time, throw the rope onto the hook you will find an orange handle. As you can't change characters in this location, you need to use one of the items mentioned above. Equip one of them, pull the handle and pick up the treasure.
As you're escaping Mount Doom, just place yourself in the middle of the road and keep jumping to automatically pick up the chest. It's inside the cave, so if you happen to miss it, press ESC and return to Middle-earth (there choose to leave the level). Afterwards choose Free Play in Mount Doom once again. That way you won't have to repeat the whole level from the beginning, which will let you focus on grabbing the treasure.