The National Palace of Sintra, in the old town, has been a UNESCO World Heritage site since 1995. One the guided tour walk where Portuguese monarchs once did and learn about how they lived in the Middle Ages.

Visit the Coats of Arms room and view the largest collection of Mudéjar Azulejos (colored glazed tiles) in the world. Check out the impressive architecture and the 33-meter-high chimneys, built in the 14th century to keep the kitchens smoke-free.

As a bonus, Sintra is usually cooler than Lisbon, so it's a nice escape from the heat!