Unlockable: New Game
Start a new game after defeating the final Boss and immediately open up the upgrade menu -- all your old upgrades will still be there but won't come into effect until the sand army is released (when you can first start upgrading your prince). With this information you should be able to go back through and get anything you may have missed first time round.
Search the following locations to find all ten Sarcophagi. When you get near a Sarcophagus, you will hear a sound and see their blue orb glow.
Near the exit of "The Stables", climb to the top of the octagonal-shaped room.In "The Fortress Courtyard", jump to the platform beneath the circular switch you have to activate to progress through the level.
After the spiked floors in "The Fortress Courtyard", drop off the main path. Slide along the series of ledges to reach the Sarcophagus.
Near the exit of "The Prison", activate the switch to open the gate. As you quickly run past the traps to roll under the closing door, and before you exit the prison, go to the right to climb the wall. Then, go right again.
At the start of "The Sewer" when you are climbing the frozen water spout, jump towards the beam. Use the frozen water spout again to reach a second beam above the first one. Then, jump over a gap to reach the Sarcophagus.
Near the exit of "The Sewer", walk along the side path on the left. Freeze the water to swing across the gap and reach the Sarcophagus on the other end.
Enter a room in "The Royal Chambers" with a water spout on the left with switches to its sides. Freeze the water and use it to jump towards each switch. The Sarcophagus is behind a door that opens after activating both switches.
In the middle of "The Royal Chambers", you will reach a room with a waterfall on both sides and two accompanying water spouts. Freeze the water, jump up to the second water spout, and use it to jump towards the waterfall. While in mid-air, quickly unfreeze your ability so you will go straight through the waterfall and land on a ledge. Then, climb up to find the Sarcophagus.
Near the exit of "The Royal Chambers", jump on a ledge to the right of the water wheel room. Continue on the ledge without freezing the water. Then, perform a wall run up from the ledge to reach the wall. Jump off the wall to reach a pole. Jump off the pole to another ledge, and move left across it to find the Sarcophagus.
At the beginning of "The Throne Room", climb the wall on the left until you reach the balcony to find the last Sarcophagus.