National Palace of Mafra
Built in the 18th century at the order of of King John V of Portugal, the National Palace of Mafra is a former royal summer residence, basilica, and library, and one of the most culturally significant buildings of the Portuguese Baroque period. Today, it's a UNESCO World Heritage Site and one of the best things do near Lisbon, as it is located just 30 km away from the Portuguese capital.