I have seen videos of travels to Sintra and read reports. A number of people mentioned it was a fun hike up to the castles/palaces. We rode the train from Sete Rios station until the end, and just assumed we could walk around and walk up. It was way different than expected.
We ended up hiring a guide - which I am very hesitant to do = but this worked out well. There was a winding drive up to the old city, and then the castles. Most of the roads were narrow one way roads without sidewalks. There was a lot of traffic from all the tourist guides in cars, tuktuk's, jeeps, and even intercity buses. Pedestrians walked along the side - not even on sidewalks - just on the should have the road or in a gutter of sorts most of the time, with a bus a foot away. I can't imagine trying to hike that.
There are the two famous places - the Pena Palace (Yellow/Orange) and the stone slightly crude Moorish castle. But there are several others too. And it seems all of them require you to buy tickets to enter. And those tickets should probably be bought in advance. In particular the Pena Palace and the Quinta da Regaleira had crowds of people near their front gate with people swarming into the road way. It was kind of nuts.
The old city with the Lord Byron cafe is a nice hilly medieval design which is cute, albeit super touristed up, and so crowded you can't get a good picture of anything.
We ended up just seeing the palaces from various vantage points without entering. And even that was somewhat tiring and stressful. We enjoyed a trip out of Sintra down to several seaside resort areas and had some great fish dishes at a scenic spot. Overall we were happy but Sintra itself was not very enjoyable at all - think Venice crowds on narrow traffic lined hilly roads. A lot of time sitting in traffic. Apparently there is not off season to this area either. I wouldn't go back. But you could buy tickets in advance, and then take an unber up the hill perhaps.