In the level "Duke Nukem's Titty Club," go through the Restrooms halfway and on the left side take a door with an "EXIT" sign over it. Head upstairs until you reach a small room. At the end of it a desk with a surveillance monitor on it and, next to it, the "Totem" from the movie "Inception" spinning without stopping.
There are 28 Ego Awards. There are a few extra rewards that are gained automatically by playing through the story. If you get all Ego Awards during your first playthrough, you will unlock the "I Am All That Is Man" achievement in the "Blowin' The Dam" chapter.
You can find poop in toilets. There is some poop in the toilets in the room where the game begins, in the "Duke Lives" chapter. Grab some poop from a toilet and throw it to earn the "Turd Burglar" achievement.
In Part 2 of "The Forkstop" chapter, you will come across a large area where you must turn the power on. After killing a few waves of enemies, use the forklift and climb the wall to your right. Grab the heart barrel, and go to the other side. Place the barrel on the blue heart drawing on the ground to open the secret room and unlock the "Companion Barrel" achievement.
In the "Damn It’s Late" mission, you will see an actor yelling at someone once you walk off the set. He says things like "You're a nice guy, you really are but you're a fucking amateur" and "Do you want me to trash you're mics? DO YOU?".This is a reference to when Christian Bale yelled at a crew member on the set of Terminator Salvation.
In the "Duke Nukem's Titty City" chapter, drink all nine beers in the strip club to unlock the "Party Animal" achievement. One beer is behind the locked door; the code is "4768". Get all the other items required to continue the chapter. After giving the items to the stripper, you can enter the last room.
To defeat the energy leech, keep the freeze ray on you from the previous level. As soon as the fight scene with the leech starts, it will slam its body on the sea bed. When this happens, zap the leech with the freeze ray and the leech's energy will be reduced down to the red, so you will only need 1 hit with the RPG to finish this beast off.
In "The Shrunk Machine" chapter, near the start and after entering the first large room, straight ahead across the room are some double doors. Look at all pages on the calendar on the wall behind the double doors to unlock the "I Need A Date" achievement. There is another calendar in Part 2 of "The Forkstop" chapter on the wall in one of the rooms.
A great opportunity to kill three aliens at once is early on when you must destroy the large ship with the laser gun on top of dukes building. After firing on the ship twice, two alien carriers will take position in front of you. Immediately fire at the ship (not the aliens) and they will all blow up with the ship.
In the "Titty City" chapter, there are two ATM machines; one near the bar and one near the slot machines in the game room. Go up to one and use it. It will say "card read error". Press action again and it will say "Feed me a stray cat" --a reference to the scene in American Psycho where Patrick Bateman tries using an ATM machine and it appears to him to say "Feed me a stray cat".
Pinball games are located in various locations throughout the game. The first pinball game is found in "The Duke Cave" chapter. To speed things up while playing the pinball game, get at least a x2 multiplier without wasting a ball. Hold the top left flipper and bring the ball up there. Then, immediately release the flipper and let it hit the three panels. If the ball is getting too far on the right side, you can tilt the table to the left. Repeat this method without using the tilt ability more than once every five seconds. If you use it too often, you will lose control over the flippers.
At the start of "The Duke Dome" chapter, you will get an infinite amount of explosives when entering a building. Throw a trip mine directly on one of the enemies to earn the "Sticky Bomb Like You!" achievement.
After loading up the Chapter called "Duke Nukem's Titty City", go behind the Bar on that level to find a closed Door that requires a code to be opened. The Code is: 4768. Inside you'll find a beer, which is part of the "Party Animal" achievement and a porn film starring the Dukester himself.
At the start of Vegas in Ruins, the 'power armor' in the armor crate at the beginning of the level is none other than that of a Spartan -- and Master Chief's classic green and orange color scheme, no less.
The first helmet is at the very start of the "Vegas In Ruin" chapter, in the back of a vehicle. The second helmet is at the start of "The Hive" chapter; turn left after going through the first alien hole to find it. The third helmet is located in the "Blowin' The Dam" chapter; after planting the first bomb and fighting two octopuses, swim to some debris to your left to find it and unlock the "Bucket Head" achievement.
In the "Duke Nukem's Titty City" chapter, play a game of air hockey in the game room at the back of the club. However, rather than focusing on scoring, try defending your goal as best you can. The rest will happen by accident since the opponent is very weak.
In "The Hive," after opening a couple alien doors, you will encounter a room with tons of spore-looking things near the doorway. Take a left, then turn on your Duke Vision and crouch, under the branch like things. Then, at the end of the cave you ducked through, there is a dead EDF marine. Walk up to him to have Duke say "That's one dead space marine." His helmet looks a lot like Isaac Clark's helmet from Dead Space.