Enable Cheat mode:
Beat Drek, then do one of the combos below at the "Goodies" menu to unlock that cheat:

Big head mode for Clank:
Flip Back, Hyper-Strike, Comet Strike, Double Jump, Hyper-Strike, Flip Left, Flip Right, Full Second Crouch.

Big head mode for enemies:
Stretch Jump, Flip Back, Flip Back, Flip Back, Stretch Jump, Flip Back, Flip Back, Flip Back, Stretch Jump, Flip Back, Flip Back, Flip Back, Full Second Crouch.

Big head mode for NPCs:
Flip Left, Flip Right, Flip Back, Flip Back, Flip Back, Comet Strike, Double Jump, Comet Strike, Hyper-Strike.

Big head mode for Ratchet:
Flip Back, Flip Back, Flip Back, Full Second Crouch, Stretch Jump, Full Second Glide.

Temporary invincibility whenever health is full:
Comet Strike, Comet Strike, Comet Strike, Comet Strike, Flip Back, Full Second Crouch, Flip Back, Full Second Crouch, Comet Strike, Comet Strike, Comet Strike, Comet Strike.

Mirrored levels:
Flip Left, Flip Left, Flip Left, Flip Left, 3-Hit Wrench Combo, Hyper Strike, Double Flip Right, Flip Right, Flip Right, Double Jump, Full Second Crouch.

Trippy trails:
Wall Jump ten times, Double Jump, Hyper-Strike. 

Infinite Bolts

Go to Kerwan (the planet that you get the HeliPack on) and go to where the first Gadgetron vendor is. If you look down in front of you, you will see some ships going into a platform. Glide down into the opening below. You should be able to get a gold bolt and there should also be a bunch of other bolts there. You will notice a bunch of ships zooming past you overhead. Jump toward any of the ships and use an Upward Slash with your wrench, like a hammer. If you have good aim, you should hit one of them. Since there is an unlimited supply of ships, that means an unlimited supply of bolts. You can also use weapons to destroy the ships. Take the taxi back up when you're done.

Thanks to Revolution readers Andrew Dickinson, Martin Brodeur, and GaMeMaStEr!