IQ Puzzler Pro

The SmartGames IQ Puzzler Pro is a compact, single-player brainteaser that features 120 progressive challenges ranging from beginner to expert. Suitable for ages six to adult, it combines logic, spatial reasoning, and concentration by requiring players to fill empty grid spaces using colorful, oddly shaped puzzle pieces.

I've actually got one of these sitting on my desk. The puzzles get genuinely hard, so kids grow into it instead of out of it. Perfect for those who like a challenge!