You will probably want a sensitive barometer to act as a variometer- determining rise/fall through change in baro pressure. And a 3D magnetic compass will not only detect heading, but pitch as well. This is handy because the accelerometers may have difficulty with accuracy when there's acceleration AND tilt involved. Due to difficulty in getting absolutes, a rate gyro, which accurately detects rotation in 1-3 axes, is often helpful in stabilizing the aircraft which requires fairly quick responses.
This is a quite a difficult project.