At only 40 minutes via train, Sintra is a perfect day or half-day trip from Lisbon. The Rossio train station is city-center, and it's only €10 round-trip. Sintra is a charming town filled with castles, gardens, palaces, and beaches. There are so many possible things to do, and it can be hard to decide what looks the best. I'm here to tell you that the Quinta da Regaleira is the absolute must-do.
The Quinta da Regaleira (vaguely translating to royal estate) is around ten acres and features a palace, a chapel, and the most charming park that I wish I lived in. There are lakes, grottoes, fountains, caves, and tons of greenery.
You should absolutely grab yourself a map when you enter. There are a few hidden secrets that the Quinta has to offer, and you really don't want to miss out! The first is the Initiation Well (Poço Iniciático), which takes you down an elaborate spiral staircase into the caves beneath the property.
The cave system has three sights, and only one of them offers an exit. First, there is an unfinished well called the Poço Imperfeito, and unless you want to rock climb (which I absolutely did!), there's no way out. Next, the cave system puts you behind a waterfall, which offers beautiful views of the rock and pond. Finally, the exit is a Grotto with the coolest rock formations.
The other, lesser-known cave system starts at the southwest corner of the property. Don't miss it!
The third main must-do at the Quinta de Regaleira is a walk through the palace. It has astonishing architecture and mosaic tiling on the floors, and views from the turrets are exceptional.
From here, you can see parts of the town of Sintra and both Pena Palace and the Castle of the Moors up on the hill.
I went to the Quinta de Regaleira three times with my three different guests. I loved it each time. There was always a new nook or column I had missed before, and it offered such great opportunities for some portrait photography!
Finally, make sure to save some time for the town of Sintra on your visit. There are plenty of souvenir shops, cafes, and dessert options to round out an excellent day trip. The train back to Lisbon runs every 20 minutes during the day, so you'll have no problem getting home whenever you're done exploring.