Scrabble? Various
games that don't involve physical activity, really.
I can break down a system to analyze trends and learn how to
manipulate them to maximize my results. I once "beat" one of those
dumb Facebook games by buying and selling the right items until I
got to the top of the leader-board. I have completed two more such
games, to the point where no more progress could be made. All of
this, of course, without spending any real money for those silly
in-game purchases.
In case you're curious, the amount of time required to beat any of
these games requires dedication well past when the game stopped
being fun, until a few short months before the game is going to be
cancelled.
I'm not good at taking things as seriously as they are presented; I
tend to take serious things comically, and silly things very
seriously.
I am also pretty decent at keeping to a budget and keeping my
credit card paid off.