Sintra & Cabo de Roca
Do-able in a day trip as I did from Lisbon, although well worth a few days stay alone, especially if you want to get out of the crowds and explore the town, is Sintra & Cabo de Roca. You can get to both by an Uber or a tour company from Lisbon, or to Sintra directly, via train (~1 hr ride) from the city centre, which is your cheapest option, though Uber is likely your quickest.
Sintra is a town that the minute you arrive, you’ve feel as if you’ve stepped into a fairytale - at least I did. Hillsides covered in forest hide colourful palaces, gardens, castle ruins and monasteries that are built within, and encompass what Sintra has to offer. In my opinion, Sintra is hands down a ‘must see’ when visiting Portugal. With that said, your visit to Sintra, especially if just a single day trip, can be packed full of different castles or views depending on what you really want to see. The top 4 attractions in Sintra are the Quinta de Regaleira (home to the initiation well), Pena Palace and its monumental gardens (very IG friendly), Castle of the Moors, and the National Palace of Sintra, though there are many more to check out if time allows. To get through all of 4 of these in one day, plus spending time in the town of Sintra itself, would be a huge task (if its even possible), so if you want to see them all, definitely stay a few days — you’ll thank yourself later.
Cabo de Roca is the most Westerly point in continental Europe, and offers up some incredible views over the Atlantic Ocean below (It has been my phones background image to this day). Take a walk along the coastal edge and get swept away (sometimes literally by the winds here) by the views of the impeccable cliff landscapes, and the historic lighthouse.