one obvious possibility for a waterproof display IS a bicycle computer. It's fairly easy to drive one from an arduino, especially if you only want to display numbers in the range of ordinary bike speeds.
I went as far as a proof of concept using a speedo head to display a voltage figure, speed, and gear ratio info but I was never that happy with appearance and responsiveness. http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1244069576/45
Still, the case is the right sort of size and form factor and they usually have buttons as well as a display.