Camouflage suit
                Obtain the relic from the Temple of Khamoon.
             
                        
                Catsuit
                Obtain the relic from the Sanctuary of the Scion.
             
                        
                Cheat list
                By completing the game on Easy difficulty, you will unlock the ability to use the following cheats: 
- Unlimited breath 
- No textures 
- Glasses
             
                        
                Croft Manor music theme
                Complete Croft Manor.
             
                        
                Secret Costumes
                Collect relics in the following areas to unlock these secret costumes:
Camo Costume - Temple of Kharmoon 
Croft Manor Costume - Midas's Palace 
Golden Costume - Midas's Palace 
Doppelganger Costume - Natta's Mines 
Tomb Raider 1 Costume - Lost Valley 
Tomb Raider Legend Costume - Mountain Caves 
Tomb Raider 2 Swimsuit - Great Pyramid
             
                        
                Style Units
                Achieve 100% completion on your profile by getting all artifacts and relics in the game to unlock the Style Units level.
             
                        
                Time Trial Secrets
                Set the game difficulty to "Time Trial" and complete the following trials to earn the unlockable extras:
Unlimited 50 Cal. Pistol - Complete Peru 
Unlimited SMG + Unlimited Health - Complete Egypt 
Golden Shotgun + SMG - Complete Lost Island
             
                        
                Where SOME of the relics are
                Peru: Mountain caves:
At the end. locate the pole that moves when you swing on it, then as its swinging jump off and you should land on a ledge. Shimmy across and climb up. Eventually you will reach the top. The rewards awaits you.
Lost Valley: 
Hard to explain, er...cross the bridge above the waterfall. If you want to check, fall of the bridge and you will die in the rapids. Crawl across the wall with the machinery. Be careful, 2 bats will chase you. Pull up into a stone room and a switch will be there so pull it. A gate opens somewhere underwater. Once you have stopped the waterfall at the beginning, dive in what was once the raging river. The reward is in a cave under the water.
Tomb of Khamoon:
At the end, there are a handful of rooms. Once the gate is open, look at the gate. To the right is a room. Pull the block that was barring the way towards the gate. Climb on and grab the crack. Follow the obvious route and balance on the top of th cartouse (poles). Jump across to the room on the other side and a Mummified Cat relic awaits.
Sorry I only know where 3 are but I'll inform you if I find some more. Take care and may you have a nice day! :)