Has awesome expositions on science and art and is the former building of the Lisbon Faculty of Sciences, still showcasing a lot of the ancient furniture and the spirited large rooms reminding the small palace it once was.
It's often not very crowded and is totally the best science museum in the heart of Lisbon. The ambiance is fantastic and you can learn a thing or two in the process, all the while enjoying an old mansion from the inside.
Go on Sundays! It's free for the whole day ;)
Tu-Fr: 10:00-17:00
Sa-Su: 11:00-18:00