Strap-ya has these awesome metallic cell phone straps that look just like a Japanese license plate. You get to choose from six different designs, and then you customize the issuing city & plate number. Unlike their confusingly long, hard-to-remember American counterparts, Japanese license plates are only four digits long, so you could easily put your birthday, or the last four digits of your social, or whatever. Sorry, no vanity plates!
Product page (Japanese)