Sure, it is possible to see some of the main attractions in Sintra in just one day, although you will need to prioritize and plan your visit carefully due to time constraints. Sintra is a beautiful town in Portugal known for its stunning palaces, gardens, and historic sites. Some of the must-visit attractions in Sintra include:

1. Palácio Nacional de Sintra: This is a historic palace located in the town center. You can explore its unique architecture and interior rooms.

2. Quinta da Regaleira: This estate is famous for its enchanting gardens, including the Initiation Well and various tunnels and hidden structures. It's a mystical and picturesque place.

3. Palácio da Pena: The colorful and romantic Palácio da Pena is one of the most iconic landmarks in Sintra. You can explore both the palace and its extensive gardens.

4. Castelo dos Mouros: This ruined castle offers panoramic views of the surrounding area and is a historic site worth visiting.

5. Lunch in the town: Take some time to enjoy a meal in the charming town of Sintra. There are plenty of cafés and restaurants to choose from. And try the Queijadas de Sintra: Sintra Cheesecakes!

6. Other attractions: Depending on your interests and available time, you may also want to visit places like the Monserrate Palace or the Convento dos Capuchos.

7. Other ideas: Don't forget that Sintra is not the only palace town in Portugal - and you can skip it and skip the massive crowds. Queluz and Mafra near Lisbon have amazing royal palaces. To the south Vila Viçosa has a royal palace and castle. Alcaçovas and Estremoz also have royal palaces - as does Bussaco and Guimarães - and the Univerity of Coimbra was a palace, as was the Castle of Leiria.

To make the most of a day trip to Sintra, consider starting early in the morning to avoid crowds and maximize your time at the attractions. Parking is not easy It's a good idea to check the opening hours and ticket availability for the specific sites you want to visit, as they may vary. If you have limited time, you can also consider taking a guided tour that covers the highlights of Sintra to make the most of your visit.

Keep in mind that Sintra is a very popular destination, and some attractions will get crowded, year-round. Plan accordingly to ensure a smooth and enjoyable visit.