For romance, Pena Palace delivers.
The fairytale image of Sintra — a candy-coloured 19th-century fantasy castle with sweeping terrace views over the serra to the sea. The honest catch: it's the single most crowded site near Lisbon, with timed-entry queues, a steep climb (or paid uphill bus) from the gate, and interiors you shuffle through in a slow human conveyor. Stunning outside; book the earliest slot.