An Arduino-based device with a screen wouldn't cost too much or be too unwieldy. You could connect it to a wifi network and send payments on the go. It would also serve as a stand-alone wallet.
Alternatively, you could make a small bitcoin instance run on an Arduino-based device which would update itself with a few minutes connected to a computer. Sort of a portable wallet.
If somebody's willing to work on the software with me, I'd do with the hardware design. I design Arduino-based devices with a friend.