There are likely more secrets than are listed here
It`s a pretty small one but it`s kind of interesting. In the Intro of the game where it shows you ConcernedApe`s Image , Click on it and the glasses will pop and you will hear a duck sound.
If you click repeatedly on the "E" in Stardew Valley on the title screen, an alien opens the E like it's a door.
If a Super Cucumber is taken and used on the brown box located inside the fenced area north of the Blacksmith, The Player is rewarded with a statue 32px ??HMTGF??.