"Playing the piano is fun." is much clearer. (Pianos, by themselves, just sit there.) "The piano is fun." could be used with clarity in these cases: A: "In your opinion, which instrument is the most fun to play?" B: "I think the piano is fun." or "The piano is fun." A: "I know you play both the piano and the trombone, which instrument is the most fun to play?" B: "The piano is fun. The trombone is hard work!"

Thanks to both of you!

Yes you are right