Wahoo Blue SC Speed and Cadence Sensor
The Wahoo Fitness Blue SC Speed and Cadence Sensor allows the biking enthusiast on your gift list to track and record the speed, pedal rotation rate and mileage of a bike ride. As a nice touch, an internal odometer tracks the lifetime mileage accumulated riding the bike. Using Bluetooth, the sensor can transfer the data to several iOS devices.
It ships with its own app (available free from the App Store), but is also compatible with biking apps like Cyclemeter.
(Note: The Fitness Blue is not compatible with anything older than the iPhone 4S, 5th-generation iPod Touch or 3rd-generation iPad.)