(Post-game spoilers below)
Celeste has a few hidden secrets that aren't listed on the main menu.
In addition to the 175 red strawberries listed on the stats screen, there are 25 unlisted golden strawberries you can earn for a total of 200 strawberries.
After you've completed Chapter 8's B-side, a golden strawberry will spawn at the start of each chapter. To 'bank' the golden strawberry, you must beat the chapter without dying. You can earn 24 golden strawberries this way.
The final golden strawberry can be earned by completing Chapter 1 without dashing.
Hidden Developer Room
There is a hidden developer room containing characters from various indie games that Celeste's developer (Matt Thorson) has worked on.
- In Chapter 6, select the Reflections checkpoint to start by a broken elevator.
- Air dash upward and climb the wall to the next screen.
- Next, jump and dash up to do a boosted wall-kick off the first small ledge on the left to reach and grab second ledge.
- Jump towards the right side to find an invisible alcove to recharge your dash.
- Finally, do a dash-jump across the pit, then dash upwards into another small invisible passage.
(For a full list of characters, see Who are the characters in Celeste's developer room?)
Earn a 1-Up
You can earn an achievement for earning a 1-up by holding on to six strawberries and banking them at the same time.
(See Do points do anything in Celeste?)
This list is likely incomplete. Feel free to edit this list if you know any secrets I missed.