Free Logic Resources
The study of mathematical logic and reasoning is important not only because it provides the best possible exercise for your brain, but also because a surprising number of standardized tests have logical reasoning sections. But I think it's work working on logic skills because they're fun!
Free Logic Worksheets
When I was a kid, I loved those Penny Press logic books. I still buy them whenever I see them in an airport. I'm not big on worksheets, but logic worksheets are basically the educational version of coloring activity books.
The Critical Thinking Company sells expensive, but very good, workbooks that make excellent supplements for a wide number of subjects. They also offer plenty of free stuff. Download these free samples, and sign up for their mailing list where they'll send you free PDF pages directly from their workbooks every week.