After you fill in the grid, observe the leftmost column, specifically the top five squares of that column. Then observe the rightmost column, specifically the bottom five squares of that column.
This page contains spoilers from the 2-Tone Game. If you haven't already played the game, you might want to do so before continuing reading this page.
Sudoku is a variant sudoku.
Its first draft looked like this:
After you fill in the grid, observe the leftmost column, specifically the top five squares of that column. Then observe the rightmost column, specifically the bottom five squares of that column.
On the whole, playtesters were pretty "meh" about this puzzle, though a couple liked it.
The presentation changed during playtesting to take away a red herring: the numbers outside the grid started out with a smaller font size than the final. It looked like I was trying to fit those little numbers to fit in the width of one little square. Players thought that those numbers somehow corresponded to one skinny column of the grid, instead of a 3x3 section. Using a bigger font seemed to help.
This puzzle solves to "chiba beach", and then expects the player to enter ONJUKU, the name of one beach in Chiba. However, there are several beaches in Chiba; Onjuku is just a prominent one. So I entered helpful responses for other beaches. So if a player entered KUJUKURI, they'd see a message, "There's another beach called Onjuku."
If the metapuzzle was based on the puzzles' answers (my original intent), then it would make sense that I steered the player to a particular beach in Chiba. As it was, all this now seems a bit silly. Alexandra Dixon picked up on that.