dystopian-ish idea: a 256×256 patch of e-ink-like material sticking on top (or even replacing???) of the forearm that can dynamically display barcodes, both 1D (think of the barcode at the backcover of your book) and 2D (QR or Aztec)
(1 of 2 skeets)
(1 of 2 skeets)
Comments
by default the patch displays a checkerboard pattern to prevent accidental scanning
(2 of 2 skeets)