In Sintra, Portugal - Quinta da Regaleira

Sintra itself really has enough things for a full day. We spent 2 there. So I wouldn't try to do much of the surrounding area if you are thinking of travelling away from it.

We really enjoyed Quinta de Regaleira. I agree it is different and the gardens are lovely. We went in the afternoon after a busy morning with Pena Palace and the Moorish Castle, and it was nice just to wonder around

The colorful Pena Palace is truly stunning from the outside. (I didn't go in - I have seen enough palaces inside to satisfy me for the rest of my life. I have heard Pena is a bit of a "shrug" inside.)

But the Quinta da Regaleira was my favorite thing in Sintra - it was unique for sure.

I agree with Andrew - the Quinta in Sintra is one of my favorites. You do benefit greatly from a guide, though. And, the house itself isn’t that great, so save it for last in case you run out of energy!
Monserrat is also a favorite.

I was in Sintra last month for two days. I arrived about 10am, dropped my luggage at my BnB, and then walked to Quinta. I took my time enjoying the grounds, taking lots of photos, and also went inside the palace. I also went to the National Palace in town on that same day. If you only have one day, and it is a nice day, I'd go to Quinta and then Pena Palace (the colorful palace up the hill). These would be my two favorites. I had two days to explore so I went to the National Palace and Quinta my first day and Pena Palace and the Moorish Castle on my 2nd day. The Moorish castle offers views, but if pressed for time - then Pena Palace is more of a standout due to the interesting architecture and colors (IMO). Quinta was just beautiful and so enjoyable to explore the grounds and was definitely worth the visit. I am 58 years old and a bit overweight - but I walked all around these places. I did take the bus up to Pena and then paid for the shuttle, too. This helped a lot with all of the hills and walking I was doing. And, I walked down to the Moorish Castle and explored it a bit. I think you could easily do 2-3 of these sights if you have a full day to explore. If you could find more time, I highly recommend staying in Sintra for the night. It was beautiful after the sun set.

I, on the other hand, loved the gardens at the Pena Palace much more than at Regaleira. My favorite sight was the National Palace - went around 4-5 pm and there were only about a dozen visitors in total. At 9 am there were busloads waiting to get in.