When I was building my multiplayer Sudoku app, I had fun experimenting with how small I could pack a sudoku grid into a URL. I came up with a scheme combining bit packing with the knowledge of how the grid works. So as you move through the grid, based on the knowledge of what's been placed where and the remaining digits you have a fewer options.

Was fun, I was playing code golf with myself. Sadly seem to have lost the code and I didn't use it in the end.

It would probably have been possible to go smaller than I got it by combining it with a sudoku solver, so stop packing positions once it's solvable. But life moved on.