Explore the relation of angular velocity and centrifugal acceleration using the gyroscope and the accelerometer. (For example when putting your phone into a salad spinner)
Measure the speed and travelled height from within an elevator using the pressure sensor.
Set up a bunch of magnets as marker, so you can use this experiment to measure the speed and distance while moving along these magnets.
Measure the frequency of a pendulum on a string. You can determine the gravity constant this way.
Put your phone into a paper roll and use the gyroscope to determine its speed.
Turns your phone into a sonar to either measure distances or determine the speed of sound.
Using the acoustic stopwatch you can determine the speed of sound with two phones and a tape measure.
Measure the frequency of a pendulum on a spring.