Another interesting way to play tic-tac-toe that makes it a lot harder: There is a bag of numbers 1-9. Players take turns moving numbers from the central bag t

Another interesting way to play tic-tac-toe that makes it a lot harder: There is a bag of numbers 1-9. Players take turns moving numbers from the central bag to their own private bags, removing that number from play. Whenever a player has a subset of exactly 3 numbers that sums to 15, they win. This is isomorphic to tic-tac-toe since the magic square for a 3x3 board has rows/columns/diagonals that sum to 15.