Franklin invented swim fins and the glass armonica. (Source)
Try playing the glass armonica online.
He also invented bifocals. (Source)
Franklin came up with the phrase, "An ounce of prevention is worth a pound of cure," when he was talking about firefighting. (Source)
Franklin is the sole individual whose signature appears on the Declaration of Independence, the Constitution, the Treaty of Alliance, Amity, and Commerce with France, and the Treaty of Peace between England, France, and the United States. (Source)