I'm a big fan of Lego Mindstorms as an entry point for robotics and physical computing and even got my start in UAVs with it.
Here's the most UAV-friendly sensor yet for Mindstorms ($85):
The CruizCore® XG1300L is a digital fully self-contained MEMS gyroscope and accelerometer. It is compatible with the Lego® Mindstorms NXT kit.
It includes a signal processing unit that estimates and compensates for the most significant errors that affect gyroscopes, providing an output with minimum error. For convenience, the CruizCore® XG1300L internally integrates the gyroscope output in order to compute the angle. The angle value can be used directly as the heading measurement in navigation applications.
The CruizCore®XG1300L includes a three-axis accelerometer that provides calibrated accelerometer output. The accelerometer can be used to compute tilt angles, detect robot collisions, or to determine changes in velocity.
The CruizCore® XG1300L communicates with the Lego® Mindstorms NXT through the I2C bus. It implements a communication protocol compatible with Lego® NXT, and users can use their favorite programing tools to communicate with this device.
The sensor outputs from the CruizCore® XG1300L are near-free of errors and can be used immediately in applications.