Many people think special relativity is just for astrophysics majors, but its basic assumptions are pretty easy!
The derivation of the speed of light from Maxwell’s equations for electromagnetic waves - http://www.santarosa.edu/~lwillia2/42/WaveEquationDerivation.pdf
The Hafele-Keating experiment, demonstrating the time dilation effect of special relativity - https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment
A great video by a high school student explaining relativity - http://www.iflscience.com/physics/teenager-wins-400000-his-video-explaining-einsteins-theory-relativity