I agree that Sintra is a very beautiful place. But its beauty and its many attractions have produced a level of tourism that is currently, IMO, at the breaking point. The National Palace and the Pena Palace have extremely long lines. Though the numbers are not as high as Versailles or Notre Dame before the fire, the sensation is identical. It’s a continuous stream of people, and you just keep moving. I personally would not go to either of those places unless you can somehow find a day with no crowds. There are many other great things to visit in Sintra that do not involve huge crowds — the summer mansion of Montserrate and the convento dos capuchos (a tiny place for about 10 monks, with the walls lined in cork) are my two favorites. The moorish castle gets crowds, but not so overwhelming. It has great views and is fun to climb around. This
English language website has a drop down menu on the top left and you can get information about all of the places to visit. Totally agree with
@Vacajoe that being in the village when the tour buses have left gives you a chance to really enjoy this pretty little place, at night with the twinkling lights it is very peaceful. So I would recommend an overnight if you can swing it.