During gameplay press the following at the same time:
Alternate Way To Get Random Weapons And Items Needed To Complete Level - D+U+K+M
Random Items/Weapon - N+U+K
Refill Health - E+A+T

If you press G+O+D in the shareware version you get the message:
"Praying won't help you! Just get out there and blast everything!"
along with an in-game plug for buying the registered version.

https://www.youtube.com/watch?v=m5SLzOhnjdA
There are 7 secret bonuses you can get on every level, each giving you 10,000 points at the end of the level. Here they are:

1. Destroy all the spy cameras in the level.
2. Finish the Level without getting hit.
3. Finding every weapon on a level.
4. Getting all the merchandise in a level.
5. Destroy all the spinning guns in a level.
6. Destroy all the napalm bombs in a level.

7. Shooting all the crystal balls, and not touching them in a level. (Including the Hint Globe)

=Game Items=
Boxes
These aren't collectables, but if you shoot them they reveal a collectable. Red boxes contain coke or napalm bombs, blue boxes contain bonus prizes, gray boxes contain important items or nothing at all, and green boxes contain weapons.
Points: 100

Soda Can
This delicious soda gives you one unit of health..but, if you don't need the health, shoot the can and grab it while it flies upwards for 2000 points. The tradition of soda cans was stopped for Duke 3D because Apogee Software was afraid of legal issues.
Points: 100-2000

Turkey
This running featherball gives Duke a health point. Shoot the turkey to cook it, than pick it up and Duke gets two health points.
Points: 100

Atom Powerup
This whirling atom gives one health point, but if you pick it up at full health, you get 10000 points.
Points: 500-10,000 (When at Full Health)

Glass Orbs
These pretty orbs are cleverly hidden all over the alien worlds Duke travels.
Points: 500 (blue), 1000 (red), 5000 (green), 10,000 (white)

Duke Nukem Danger Shades
You've got to keep cool in the face of danger.
Points: 100

Duke Nukem 2 Computer Game
Maybe this disk has Duke Nukem 2 on it...
Points: 500

Duke Nukem CD "Duke in Love"
I wonder who Duke's in love with...
Points: 500

Duke VHS Tape "Why I'm So Great"
I love self-help tapes.
Points: 500

Duke Nukem Video Game Cartridge
Maybe for SEGA Genesis, SEGA 32x, SEGA Master System, Nintendo Console, Super Nintendo or TurboGrafx-16?
Points: 500

Duke Nukem Boom! Box with Motivational Tapes
Contains "Why I'm So Great".
Points: 1000

TV Set
Contains Cable TV & PPV Services: Adelphia, Armstrong, AT&T Broadband, CableOne, Charter, Media One, Cox, Comcast, Falcon Cable, Verzion, First Row, New Visions, CableVision, RCN, Star Power & Time Warner Cable. Also for Watching Duke's Numerous TV Appearances
Points: 1500

Duke Nukem Commando Phone [with Button 1 Set to 976-DUKE]
Now how come this number never became as popular as Tommy Tutone's 867-5309?
Points: 2000

Duke Nukem Actioncam with Heroic Music Audio Overdub
Do Duke's love ballads count as heroic music?
Points: 2500

Duke Nukem Brand 486/66 Localbus PC
For Playing Duke Nukem 2 & other Apogee Software Titles.
Points: 3000

Signed Duke Nukem T-Shirt
Now that shows devotion, a normal egotistical hero scattering thousands of signed t-shirts in remote and dangerous places around the galaxy.
Points: 3000

N.U.K.E.M.
These letters are 10100 points each, but when picked up in the correct order, they award a 100000 point bonus at the end of the level. If you get them all but in the wrong order, it's still a 10000 point bonus.
Points: 10,100 (Each), 50,500-10,000 (any order total plus bonus), 100,000 (correct order)

Key
This can open doors.
Points: 500

Access Card
Use this computer chip to deactivate the electric barrier guarding the exits of some levels.
Points: 500

Rapid Fire Powerup
For a limited amount of time, you can constantly fire bullets by holding your shoot key.
Points: 500

Cloaking Device
This temporarily makes Duke invincible so he can go through some force fields.
Don't worry if it runs out, as it regenerates.
Points: 500

Hint Globe
Bring this to its pedastal to get a hint for the level you're in.
Points: 10,000, 50,000 (bringing it to the pedastal)


=Weapons=
Normal Weapon
These rarely appear in the game and if you get one, it takes away any powerful weapon you have.
Points: NA

Laser
You get 32 powerful shots with this that go through anything, including walls.
Points: 2000

Flamethrower
This is one hot weapon. You get 64 shots with it, but you're going to need them if you use its special feature. By shooting down with this beast, you blast yourself up so you can effectively use it as a jetpack to get to higher areas.
Points: 2000

Rocket Launcher
This is an extremely powerful weapon, and with 32 rockets to shoot you can do a lot of damage.
Points: 2000

https://www.youtube.com/watch?feature=player_embedded&v=J86muOLDAS4
Tagline: TBA
Released: 1993 Apogee Software
Genre: 1-Player Platformer