A tone generator has a variety of uses. You can explore interference, waves, and the physics of music. Connect a bare speaker to your computer’s earphone jack and show how a speakers work.
This online tone generator from Tomasz Szynalski has some special features. You can specify and dial in frequencies to the nearest 0.001 Hz, select the type of wave form, adjust left vs. right balance, and select musical notes. Open copies of the site in multiple tabs or browser windows to run multiple tone generators at once. Look thru the Instructions to see everything this tone generator can do.