The nearest airport is in Humberto Delgado Airport (LIS), located 13.5 mi (21.7 km) from the city center. If you can't find a flight that's convenient for your trip, you could book a flight to Cascais (CAT), which is 6.3 mi (10.1 km) away.
If you prefer train travel, make your way to Portela de Sintra Station or Sintra to see more of the city.