Braid Unlockable: Alternate Ending
Collect all seven hidden star icons in the game and then collect the 8th star that appears in the final level to view an alternate ending.
Braid Unlockable: World 1 and Epilogue
Find all 60 puzzle pieces to repair the ladder in the house. This allows you to reach the attic and unlock World 1. Beat World 1 to unlock the Epilogue area.
Braid Easter Egg: Hidden Text in Epilogue
Each of the 4 first rooms in the Epilogue has an object obscuring additional text behind it. Go behind these and adjust your position until you hear a sound and the text appears. You'll need to manipulate time to read all of this text due to the opening and closing books.