Press Up(2), Down(2), Left, Right, Left, Right, Up(2), Down(2), Left, Right, Left, Right at the main menu. The sound of an elephant will confirm correct code entry. When you start another game all animals, foliage, buildings, and other objects will be unlocked.

All animal houses
Change a guest's name to Andrew Binder.

All animal shelters unlocked
Change a guest's name to John Wheeler or Adam Levesque.

All animal toys
Change a guest's name to Lou Catanzaro.

All foliage
Change a guest's name to Charlie Peterson.

All scenarios completed
Change a guest's name to Akiyama.

Animals breed more frequently
Change a guest's name to Dr Dolittle.

Blue shirts
Change a guest's name to Mr. Blue.

Brown shirts
Change a guest's name to Mr. Brown.

Endangered animals
Change a guest's name to Steve Serafino.

Money donated
Change a guest's name to Bill Gates.

Orange shirts
Change a guest's name to Mr. Orange.

Pink shirts
Change a guest's name to Mr. Pink.

Sick guests
Change a guest's name to Zeta Psi.

White shirts
Change a guest's name to Mr. White.

Yellow shirts
Change a guest's name to Mr. Blonde.