Steve Martin plays decent three finger banjo. Much more impressive was Dudley Moore. He may well have been the best Jazz pianist I have ever heard.(He worked with John Dankworth and Cleo Laine). Jack Lemon was a professional pianist and Fred MacMurry put himself through college playing the saxophone. Groucho Marx played classical guitar and took lessons from Segovia.
Did Kirk Douglas really play the guitar in "20,000 Leagues Under the Sea"? I loved that song he sang, "Whale of a Tale". Alan Arkin was in a folk group with Lee Hayes. Mickey Rooney was a dynamite drummer.
My favorite celebrity-musician sighting was on the old TONIGHT SHOW, when Maurie Wills, he played shorstop for the Dodgers, did a few fast numbers on tenor banjo. Hey, he was better than George Segal.