The Museum of Ancient Greek Instruments is walking distance to the Plaka and Roman Agora. It's located in a mansion built in the 1800's, and has over 1200 instruments.
You can not only observe the three floors (divided into four sections) of instruments, but you can listen to each sound they produce. There is a permanent exhibition and other items that go "on the road" to educate the school children with their heritage. Concerts are held to share and educate the public.