A fun part of math is just messing around with various functions. Here you can find some things you can play around with.
Differentiate a function of your choosing. See all the steps shown.
Plot a function of your choosing. Use sliders to control other variables.
Play with the relationships between the unit circle, the trig functions, and their graphs.
Move around a sound-emitting object and an observer to hear doppler shift.
Slide trains on their tracks, and watch how they move together.
Twelve fifths is almost seven octaves—and you can hear this fact.
You can plot slope fields! You can see Euler's method in action!
Estimate the (signed) area under a curve using a Riemann sum.