Codes, easter eggs, and secret stuff: This is the place to find an up-to-date listing of all the goodies the Zoo Tycoon developers have sprinkled throughout the games. Although you'll find many codes and secrets listed on other sites, this is the official list of Zoo Tycoon secret stuff.

Key:
(DD) = New to Dinosaur Digs
(MM) = New to Marine Mania

Cheat Codes Codes (also known as "cheat codes") are actions you can take that can give you the edge you need to succeed. To rename a guest, employee, or exhibit, click the desired object, then type a new name into the Info panel.

Name a guest Adam Levesque and unlock all Animal Care research programs. (Adam Levesque is the CEO of Blue Fang and Creative Director for Zoo Tycoon.)
Name a guest Lou Catanzaro and unlock all Animal Enrichment research programs.
Name a guest Charlie Peterson and unlock all exhibit foliage. (Charlie Peterson is one of the Program Managers on Zoo Tycoon.)
Name a guest Akiyama and unlock all the scenarios that ship with the game. (Akiyama is the family name of one of the test team members.)
Name a guest Steve Serafino and unlock all Endangered Animals research programs. (Steve Serafino is one of the designers of Zoo Tycoon.)
Name a guest John Wheeler and unlock all animal shelters. (John Wheeler is the Engineering Director for Zoo Tycoon.)
Name a guest Ron Propst and unlock the Dinosaur Foliage research program. (DD)
Name a guest Hank Howie and unlock all Staff Education research programs.
Name a guest Eggman and dinosaur eggs hatch more quickly. (DD)
Name a guest Dinoman and unlock all dinosaurs. (DD)
Name a male maintenance worker Bob V and all currently broken fences are fixed. ("Bob V" refers to Bob Vila, the well-known home improvement show host.)
Name a scientist Dr. Scholl and all scientists walk twice as fast. (DD)
Name an exhibit Cretaceous Corral and unlock the Triceratops.
Name an exhibit Xanadu and unlock the Unicorn. (Xanadu was Kublai Khan's summer capital in China, where he met Marco Polo in 1275. It is also the title of a poem by Samuel Taylor Coleridge.)
Name an exhibit Super Croc and unlock Deinosuchus.
Press $ (Shift+4) and get $10,000, but a random exhibit fence weakens (great time for the Bob V cheat!).

Easter Eggs
Easter eggs are surprises that you may or may not run across as you play. Many of these are date-depended, but pay attention—some relate to the in-game date and others to the real-world calendar.

On September 15 on the in-game calendar (the date of the signing of the contract between Microsoft and Blue Fang for Zoo Tycoon), a biplane with the Microsoft logo flies across the map pulling a banner with the Blue Fang logo.
On their own turf, penguins are capable of killing pretty much any animal in the zoo.
Guests entering the zoo on January 1 (New Year's Day) on the in-game calendar may be sick.
October is Dinosaur Month! Purchase all dinosaurs at half price during the in-game month of October.
On October 31 on the real-world calendar, all guests and buildings wear orange and black in honor of Halloween, a jack-o-lantern is available from the buildings and items menu, and a wicked witch flies over the zoo on a broomstick.
On October 31 (Halloween) on the in-game calendar a witch flies over the zoo on a broomstick and a jack-o-lantern is available from the buildings and items panel.
On Christmas (December 25) on the in-game calendar, Santa Claus flies over the zoo in his sleigh.

Secrets
Secrets are sort of a combination of the two, and are mostly just for fun. Enjoy!

Name a male maintenance worker George W and all the foliage in your zoo will vanish, but you'll receive $300 tax rebate.
Name a guest Mr. Brown, Mr. Pink, Mr. White, Mr. Blue, Mr. Blond, or Mr. Orange and all guests and buildings currently in the zoo change to the associated color.
Name male maintenance worker The Rock, all buildings are turned to rubble.
Name a bear Deer and the bear will immediately break out of its exhibit (unless the fence is electrified).
Place a Mermaid Statue in an exhibit or show tank and it changes into a real mermaid. The Mermaid will not appear in the adopt an animal menu; however, you can continue dropping Mermaid Statues into water tanks to "adopt" more.
Name a guest Zeta Psi and all adult male guests in your zoo change colors and become sick. (Zeta Psi is a college fraternity.)
Name a guest Alfred H and a flock of birds appears to frighten your guests.
Place a lion, a Bengal tiger, and a bear in an empty exhibit and unlock golden brick walkways.
Name a scientist Dr. Frankenstein and there's a 10% chance an egg will change its contents when the scientist visits. (DD)
Name an exhibit Copralite Corner, all dinosaur poo turns into rocks. (DD)
Name a guest Stinky and the guest causes other guests to be sick. (MM)
Name a male guest rpro and female guests will run from him. (MM)
Name a male guest Boogyman and the guest causes children to run away screaming. (MM)