If you have an old keyboard lying around and wish it were wireless, Maker DastardlyLabs has a solution.
The “HID Relay” is a small adapter that uses an Arduino Pro Mini, a Bluetooth module, a USB host shield, and a few other components to upgrade any USB keyboard to Bluetooth. DastardlyLabs has made three videos to explain the entire “Bluetooth-ification” process–which can be found below.
So far, the method has worked with all of the keyboards that DastardlyLabs has tested it on, as well as most mice (except for one “gamingish” USB mouse). The Arduino source code and build notes are available on GitHub. The HID Relay was inspired by a recent Arduino hack by Evan Kale.