Auto Shotgun in First Level  -  After you've killed the first two dinosaurs in the level, walk down the narrow passageway. When you reach the end turn to your left look up and you'll see a cliff lower than the rest of the cliffs climb up the wall directly underneath the cliff. When you get to the top you'll see a guy with a pulse rifle cannon, a box of shotgun shells. Defeat him, then enter the narrow passageway to get the auto shotgun. 

Beat the Campaigner  -  When facing the Campaigner (the final boss), take out your Particle Accelerator and start charging it. As you charge it the Campaigner should stall and stand still. Now fire it at him and change your weapon (the minigun and auto shotgun work well). Even though he has a force field he'll start laughing as he blocks your attack. This leaves him open for your second attack. Repeat this process for an easy victory. 

Cheat Codes  -  Enter these codes in the "Enter Cheat" menu. To use them, go to the Cheat Menu and enable the desired codes.

All Weapons - CMGTSMMGGTS 
Big Heads - TSHNTTBNCTPRDCRD 
Dana's Cheat - DNCHN (shrinks enemies) 
Disco Mode - SNFFRR 
Flight Mode - LKMBRD (use R and L to move up and down) 
Gallery - THBST 
Greg Mode - GRGCHN 
Infinite Lives - FRTHSTHTTRLSCK 
Pen and Ink Mode - DLKTDR 
Purty Colors - LLTHCLRSFTHRNB 
Quack Mode - CLLTHTNMTN 
Robin's Cheat - RBNSMTH 
Show All Enemies - NSTHMNDNT 
Show the Credits - FDTHMGS 
Spirit Mode - THSSLKSCL 
Unlimited Ammo - BLLTSRRFRND 

Grenade Launcher in the Ruins  -  When you get to the temple with the save point on top, save then look around. Look to your side and you'll see a ten-point Life Force. Walk toward it and you should see a sparkling path. Follow it and you'll find the Grenade Launcher. 

Skip Across the Water  -  Basically, this little trick will allow you to jump on the surface of the water repeatedly to the other side. To do this, fall off the ledge toward the surface of the water, but tap the jump button as you hit the water. You should jump off the water! If you get the timing just right, you can jump right along the surface of the water! 

Ultimate Code  -  For level select, all weapons and much more, enter the code NTHGTHDGDCRTDTRK at the "Enter Cheat" menu.