[teleforce]

Great, thanks for the suggestion.

Apparently, Lichess has 'Over the board' feature that allows you to play with your friend offline, nice!

[naet]

It also has a pretty good chess clock if you have a physical board but want to play timed.

[mat_epice]

It also lets you play an engine (stockfish) running on your phone. I play offline on planes all the time.