Pena Palace
Take bus 434 to the top. Grounds-only ticket. With 2.5 hours you can walk the full rampart loop, explore the lower terraces most visitors miss, and linger at the western viewpoint with Atlantic panoramas. We sat at the western viewpoint for 20 minutes watching the morning mist burn off the hills below — something you'd never have time for on a day trip.
The park trails below the palace are worth 30 extra minutes. We found ancient fern trees taller than us, a hidden fountain covered in green moss, and saw exactly two other people the entire time.