I was staying in the Sinatra mountains so a day trip to Sintra was just just 10 minutes away. After doing my research on what to do in Lisbon, a day in Sintra as at the top of the list. Given I was staying in the Sintra mountains, it was just 10 minutes away. However, for those staying in Lisbon, I would still recommend this as a day trip. It is such a quaint little town and has several attractions to see, the National Palace of Pena, Quinta da Regalia, The Moorish Castle and the National Palace of Sintra. Unfortunately, we did not get to tour much beyond the National Palace of Pena because it was Christmas Eve and everything was closely early for the holiday. I had heard from more than one person that the gardens at Quintal da Regalia were beautiful and to be missed, so I'm bummed we missed them! However, we did get to see The Pena palace and it looks like a set from a Disney film.
We then had a late lunch at a little restaurant in the middle of the square right by the National Palace of Sinatra. It seemed really touristy so we didn't expect much but it was one of the places actually open. We were pleasantly surprised by how good the food was! The Traveling Hillbilly recommends the vegetable soup. It was more of a potato soup with vegetables, great comfort food on a cool day.
I hope you all get the chance to visit one day.