Best way to see Sintra?

I can not imagine any possible need for a "tour". Take the train. It's super, super easy as could be.

You have the Rick book, right? Every detail you could possibly need is in there.

Are you staying in Lisbon? it's super easy to get to Sintra by direct train from there - there are direct commuter trains from Rossio Station or Oriente several times an hour.

Once you get to Sintra, there are a lot of things to see and it's a very busy place with lots of tourists. Most people take a bus or "tuk tuk" (mini-taxi) up the steep hill to the palaces and castles (serious hikers might hike up and/or down). The 434 bus does a "loop" from the train station up to the palaces and back, but it's a busy bus. I used it. One suggestion: buy your tickets to the Pena Palace online ahead of time - otherwise, you'll wait in long lines at the Pena entrance. I didn't bother seeing the inside of Pena Palace (costs extra), I just wanted to see the outside and the grounds around it. I wish I had gone to the Moorish Castle first (you can buy tickets for Pena there too) then hiked up a short walk from the Moorish Castle to Pena. The 434 buses stop at Pena and wait to go back down the hill; it's a convenient place to get on.

I didn't see everything in Sintra - there is a lot to see and it takes time to get from place to place - but my favorite was the Quinta da Regaleira, which is a unique garden complex down by the town, about a 10-15 minute walk from the center.

Do you recommend buying tickets for Palace Pena and Sintra both online? The website is not user friendly!

I didn't visit the Sintra palace, so I don't know how crowded it is.

I would say the best way to visit Sintra is NOT just for a day trip, go and stay a while. You don’t say how much time you have. When I was in Portugal in 2017, I arrived at the Lisbon Airport, then went directly to Sintra for 2 nights. Great town, various sites each of which merit a half-day, some excellent restaurants and bakeries and interesting shopping. If you do go just for the day, just take the train, and try to make it a long day. Also, I understand that Monday is the most crowded day, since museums in Lisbon are closed on Mondays, so people go to Sintra then. As for Pena Palace, I went on my own on bus 434, first thing in the morning, as the palace opened. Went through the interior, came out and there was a tour of the extensive grounds available, for 3 euros I think. It was by vehicle, sort of like a very extended golf cart. Turns out at that early time, I was the only passenger and the young fellow who was driving and I had a great time for much more time than usual, stopping at other buildings, horse facilities, gardens and viewpoints. I also really enjoyed the Quinta da Regaleira and wished I could have spent more time there, as I went the first afternoon after my arrival for plane, and both myself and my camera battery ran out of energy during the afternoon.

Staying a night or two in Sintra isn't a bad idea if you have time. It's a quiet town at night (I left late in the afternoon on the train back to Lisbon, and by then it had quieted down a lot). It is hard to see absolutely everything in a day, though it could mostly be done. Personally, if I were to do it again, I'd see the Pena Palace (still not go inside) but not waste my time in the park around it, which I didn't find especially worthwhile. I did it on foot. Also, the Moorish Castle ruins were OK but not amazing. More interesting from a far than up close!

I do regret not seeing Monserrate (or the grounds, anyway), which was a bit out of town. I have been inside enough palaces and don't really feel the need to see many more. The outsides of these places are much more interesting to me than the insides - but our interests as travelers vary person to person.