I propose that the reverse side of $1, $5, $10, and $20 bills be redesigned, to include useful information. Consider adding a metric rule printed along one side, and an inch rule printed along the other.
For the remainder of the area, add such items as metric to US conversion tables, cups per pint, pints per quart, etc. tables, tipping schedules, time zone offsets, simple math and geometry formulas (area of a triangle and circle, simple interest) and other general information that is useful in everyday life.
Printing of these features could still be sufficiently complex to provide anti-counterfeit protection, and even artfully done. The cost of this should be minimal, and yet it would provide a welcome utility that is as close as your pocket.