You can have different things in a secret, and it doesn't even have to be an ending.
The objective of the secret would be to give the player a little bit of extra and a notice that its a secret.
For instance, if you have a level with difficult jumps, you can put a high jump powerup in a secret area. If the player finds it, the level suddenly becomes alot more easy.
Another idea is to create an image that looks very nice, or even a quote that has a deepfull meaning. If the player finds it, thats his reward.