https://www.parquesdesintra.pt/en/parks-monuments/park-and-national-palace-of-pena/
Keep scrolling down to find these untimed tickets that allows visit to all those beautiful colorful terraces (all except one) and even cafeteria
You do not have to stand in any lines.
You are not slave to any time slot.
Never heard them getting sold out (but in theory possible)
Some opinionated advice:
(Few may wish to differ)
You can wait until previous evening or sunrise that same day to decide after checking for wind/rain at Sintra (which could have very different weather than Lisbon)
Ignore all travel guides and blogs, I think oft repeated advice on this forum is golden. Just take Bolt from your hotel to the main gate of Pena
It's actually cheaper for family of four and worth the convenience for party of twoWorth spending 3 euro to take green bus to avoid climbing the steep hill from the main gate to the castle (depending on your fitness level and time availability). Tip: if there is line to get into this small bus, volunteer to stand and get in right away. It's a 5 minute ride.
It is easier to get Bolt from the Pena main gate than it is to get from Moorish Castle main gate because of the one way nature of the road.
Many folks prefer Regaleria over overcrowded Pena Palace. Some like Monserrate Palace too.
6 Some prefer to Bolt over to Cascais and have evening meal there before catching train (or Bolt) back to Lisbon.
Reposting above summary due to many PM requests on the same topic on this forum as well as others.