Basically, some levels, aside from whatever hidden exits they might have, they could also house a password, which would be hidden somewhere in the level
Clicking the password link would prompt you for a password, and after you typed it you would get your reward.
Ideas for possible rewards:
-The new cheat codes (check the string replacement thread)
-Artwork (either fanart, or some new official Rao artwork I guess)
-Hints about other levels
-General sillyness
As of now, the only level I know of ready for the system is one of my own (Lusco-Fusco). Since it's a quite easy world 1 or 2 level with a very badly hidden password, the reward should be minimal (a joke would fit in quite nicely with the level). The rewards should be better as the game goes on.