KnaveOfAllTrades's idea of learning demographic & economic (GDP and its component parts) statistics of various places has occurred to me as a candidate for a useful Anki deck, so I second that.

Knowing some mathematical constants to a few significant figures can be useful. Memorizing √10 = 3.16 lets you interpret midpoints between ticks on a logarithmic scale, and √2 & √3 are the lengths of diagonals of unit squares & cubes. And knowing all three roots makes it easier to guesstimate square roots in general, using the √(ab) = (√a)(√b) result for non-negative a & b. Likewise for e.g. exp(2), exp(3), ln 2 & ln 3. The 68-95-99.7 rule should go on the list as well.