Is playing cards a problem, even whne absolutely no gambling is involved?
If so, what issur is it?
Is playing cards on Shabbos an issue?
There is no prohibition in playing cards.
It is likewise permitted to play cards on Shabbos, though sorting cards can be an issue of borer, and if one is not expert in the details it is quite easy to inadvertently transgress this melachah.
Although cards and other games are generally permitted (though each game must be considered independently), this is not necessarily the most appropriate way for somebody to pass his time on Shabbos, and the day should generally be reserved for spiritual activity. However, it is sometimes appropriate to play, such as when looking after children and so on.