Mad Cows:
Complete all three shotgun challenges with a "Gold" rank.

Nuclear Bomb:
To unlock the Nuclear Bomb, complete all ten deathmatch challenges. If it does not work with all bronzes, try to get silvers and golds.

Teleporting:
In levels where there are objects floating in the air, if you teleport onto them sideways, there is a chance that you will actually teleport inside the object. This is especially true for the green air ships.
Information in this section was contributed by Stephenguard1966. Hint: Level generator:
Not only can you type in numbers to generate levels, but you can also type in words and it will convert it for you.