How about doing a WiFi repeater system for cavers or for large areas.
It would be very useful to be able to put wifi repeater nodes along a path so that you could always have a connection regardless of where you roam around inside an enclosed space.
Another project would be using either laser range finders or ultrasonic devices to map the walls of a space in high levels of details.
It would be very useful to capture the full three dimensional volume of a space with a little automated gadget.
