Self-Reference Puzzles
As an undergraduate, I stumbled upon Jim Propp's Self-Referential Aptitude Test and spent a couple hours working on it. That experience stuck with me and I have since made smaller-scale versions and variants for students. This is a hard activity to pin down to an age group, but it can skew a little more abstract and occasionally arithmetic, so I'd peg it around 7th grade and up but have definitely had some luck with advanced elementary school children.