Among a lot of useful stuff like 180 degrees servos and other ready-made pan-and-tilt camera mounts, you'll find plenty of interesting parts and sensors.
The sensor that I would like to present here is the Devantech Ultrasonic Range Finder SRF10. It has got a I2C bus, is small in dimensions (32 x 15 x 10mm) and senses with 3-4 cm resolution in the ranges from 0.03 to 6.00 m.
I do not have sufficient electronic expertise to tell how such sensor could be applied easily, but I feel such sensor would be interesting for automated landing (eventually take-off) procedures. I hope some bells are ringing and someone could give insight as how this sensor could be implemented.And what about a longer range ultrasonic sensor for detecting other flying objects like real scale flying vehicles???