Sintra is a short train ride or drive from Lisbon and holds several architectural landmarks including the Pena National Palace, the National Palace of Sintra, The Moors Castle. It's also a botanical hotspot thanks to King Ferdinand II who brought trees from all over the world to be planted there (including Sequoias and Redwoods from the U.S.!). A ring of cork trees was planted surrounding the base of the mountain because they are very slow to burn and were intended to keep enemies from invading with use of fire. Today, Portugal produces more than 50% of the worlds cork and Sintra tourist shops are full of anything and everything made from cork! Several movies have been filmed in Sintra but most famously a thriller film staring Johnny Depp called The Ninth Gate.
We were given a great tip by a couple from our African Lisbon tour to pay the nominal fee and have a tuk tuk take us to the top of the mountain to tour Pena National Palace. Then after touring the palace (and taking in the amazing views), we walked back down the mountain and enjoyed a delicious lunch of tapas & porto at Tascantiga followed by Pasteis de Natas (the best custard tarts that you can find everywhere!).
Definitely recommend taking a day trip to go see the sites in Sintra! Porto was also recommended to us several times but we were doing our best to take things slower instead of running around seeing everything possible... so next time!