Sintra is an excellent day trip from Lisbon, with many tourist attractions, souvenir shops, and delicious eateries. My best recommendation for visiting Sintra is to see the Quinta de Regaleira. It's close to town, beautiful, and filled with unique sights. However, the most popular destination in Sintra is definitely Pena Palace. It is considered one of the Seven Wonders of Portugal and sees nearly two million visitors a year. What started as a chapel in the Middle Ages became a monastery in the 14th century. The monastery was destroyed in the Great Lisbon Earthquake of 1755, but the chapel survived due to its marble construction. In 1838, King Ferdinand II acquired the grounds and commissioned a palace to be his family's summer residence. It was completed in 1854.
The castle sits at the top of a hill in the Sintra Mountains and overlooks the city, surrounding forests, and the Atlantic Ocean in the distance. More than the palace itself, I was impressed by the outstanding views. Touring the palace offers many shots of these picturesque landscapes through the building's windows and from multiple outdoor areas.
If you are interested in seeing Pena Palace, keep in mind that it is pretty difficult to get to. You'll need to take a pedicab for around €20 up to the base of the park (there are people waiting right at the train station), buy tickets for a specific time slot, and then take a bus up to the base of the palace. The surrounding grounds are beautiful, but I'd suggest walking back down after the palace tour rather than hiking up when you have a time slot to catch. Be sure to save yourself a solid hour if you book tickets online because the drive and lines will take a while. The views from the castle were amazing, but it was so overcrowded that we were shuffled through the building in a cramped, slow-moving line. You will also find yourself waiting at the castle's many photo-op locations for your chance to pose.
I personally would not go through the effort of getting to Pena Palace again. For people visiting Sintra for a day (or half-day) trip, I would recommend the easier commute to the Quinta de Regaleira. You can see a beautiful castle, walk the grounds, and have time to explore the town. If you do decide to trek to Pena Palace, don't take the bus back down the hill!
If you're able, I absolutely recommend you walk back down through the gardens on your way out. There are a few hidden gems in the way of fountains, small waterfalls, rose gardens, and castles in the middle of ponds. If the weather permits, definitely save a little time at the end of your visit to walk through the park.