I don't think knights and dukes just stared at walls all day. Where they're any board games? Any sports?
During times of peace, medieval nobles devote their time hunting, sometimes leaving for far away hunting grounds and hunts that go on for days. If they season isn't fit for hunting, they spent their times either gambling as well.
If you're looking for a specific board game played during those times, there is "Alea evangelii" a tafl game which (according to Irish manuscripts dated to be written in the 11th century) was created for King Athelstan by.
hi! you might find something of interest in the FAQ (see link on sidebar)