I built Fun with Waves to explore the Playdate synth functionality. It lets you create an arbitrary number of overlapping sine waves and both see and hear the results.
Press A to add a wave. Press B to delete the selected wave. Press up and down to toggle through the waves. And use the crank to change the frequency of the selected wave.
Have fun making horrible noises!
If you don't have a Playdate, you can download the Playdate SDK and use the simulator to play this game.