If you like imaginative/challenging math/logic/CS problems, take a gander at FiveThirtyEight's The Riddler.
FiveThirtyEight is Nate Silver's
website that does precise, academic, and statistical commentary on a variety
of issues. Their
The Riddler column features hard and interesting problems
on a weekly cadence.
In the past, I have solved several, with at least one of my solutions being featured with a shout-out. My solution with code is on GitHub. This particular problem was a fun little geometry problem which I opted to solve via simulation and numerical geometry, though others found an elegant analytical solution.