Ultimate Tic-tac-toe — two players, one PIN

Nine tic-tac-toe boards arranged in a 3×3 grid, and one rule that turns a solved children's game into something you can actually think about: the square you play in decides which board your rival must play in next. Win a small board and it counts as one square on the big one; three in a row on the big board takes the match. So a move that wins you a square can hand your opponent the free run of the board you needed. It never draws the way plain tic-tac-toe does. One hosts, one joins with the PIN.