From the main screen, click EXTRAS then REDEEM CODE.

I was able to find these codes on multiple websites:

ThisIsAnAwesomeCode - this code provides items from Learn to Fly 2

IBelieveICanFly - this code gives you 250 BP

I haven't seen any of the following codes online so I assume they're unknown (or at least little known):

ltf3mailinglistbonus - I believe this code unlocks the Capt. Supporter body

BirdIsTheWord - this code gives you 250 BP

WhoSaysWeCantFly - this code gives you 250 BP

TimeSureDoesFly - says it's a Fast Forward code, but I got a "code invalid error" when I tried it

UPDATE:

After receiving Freefilly's comment, I looked into it and found that the debug menu has been removed in the latest update and is no longer accessible. The other codes listed above should still work.

引用自 Sitizyn：

While this code was fun to play around with, I must warn you. It will, without a doubt, ruin the game for you. If you are really enjoying the various challenges then I'd suggest you skip this one.

This code allows you to unlock all items, win the game by clicking a button, and may give all medals by clicking a button though I wasn't able to test that one since I already had them all. It will also give you unlimited cash and BP.

What it will not do is give you unlimited special powers or rare points (though you can get them by repeatedly "winning the game").

Steam achievements WILL unlock while using this. Very useful for the "I can never count that high..." achievement.

enableTestMenuK8DH76G5DGh3GH5g3bB5Gh4 - enables the in-game debug menu.
To access the menu, start a game in any mode, and press F2 while at the main shop screen.
You'll regret using it half an hour later.

Pressing F8 will bring up the games' log file.

Pressing F7 will display FPS, memory usage, and some other info.