How about a nice game of chess?

The engine is Stockfish.js 19 in its lite version, compiled to WebAssembly. It calculates entirely in your browser; no move leaves your device. chess.js checks the rules. Your browser remembers the current game, so you can carry on after a reload.

Levels one to six limit search depth and playing strength; from level seven the engine gets thinking time. At the highest level it is stronger than any human. Good luck.

Stockfish is licensed under the GNU General Public License v3. The source code is freely available from Stockfish and Stockfish.js.