How can I measure the RPM of a Frisbee?
Off the top of my noggin here's an idea. Build a small tone generator and tiny speaker onto the frisbee with the speaker around the edge (balanced mass on the other side of course).

When you spin the frisbee you could record the sound being made and it will have doppler features depending on how fast it is spinning. If you can analyse the data recorded you should be able to tell the rotational speed.