While it is physically possible to visit both Sintra and Cascais in a single day from Lisbon, it is generally discouraged by travel experts because it forces you to rush through two of Portugal's most significant heritage sites. To attempt this "marathon" day in 2026, you would need to start at 8:00 AM in Sintra to beat the massive crowds at the Pena Palace, spend roughly 4 hours exploring the town's hills, and then take the 40-minute bus (Route 1623) or a taxi across to Cascais for a late lunch and a quick stroll along the coastline. You would likely miss the mysterious Quinta da Regaleira or the Moorish Castle in Sintra, and you wouldn't have time to properly enjoy the beaches or the "Boca do Inferno" in Cascais. Most visitors find that Sintra alone requires a full 8 to 10-hour day due to the steep terrain and frequent shuttle bus wait times. If you are determined to do both, the most efficient method is to book a private guided tour that handles all transportation and utilizes pre-booked, timed-entry tickets to minimize the time spent in queues.