After reading many posts, I am thinking of visiting Sintra early morning and leaving later in the evening to Lisbon. I would like to visit the exterior of Pena Palace. Then go to one or two other places. Does anyone have a day plan they would like to share? I am open to any plan as it all looks so beautiful. I am not sure what I should skip and what I should keep. I would really appreciate some tips for Sintra.

Here are a couple of good threads to get you started. We are planning a trip there next spring and I found all of these helpful for Sintra:

Impossible choice: Pena Palace or Quinta da Regaileira ... If you can only do one!

Lovely Lisbon: Fresh Seafood, Custard Tarts, Miradouros, Hills and more Hills!

PORTUGAL: Lisbon and Sintra; a photographic trip report

We took a day trip from Lisbon to Pena Castle via train. Super fast and easy. We got there later in the afternoon so only had time for the castle but I would imagine if you got there early in the day you would have time for either the opposite older castle (I forget the name) or be able to wander the grounds. Just don't try to get the shuttle to the castle too late, they were turning people away when we got back. Also if you are waiting for the train go to the little café/pub and grab a Pastel de nata and beer or coffee.

Pena Palace exterior, then Quinta de Regaleira and you will see the exterior of the Sintra National Palace in the center of town when you arrive. The best interiour is Quinta. Not essential to see the inside of Pena Palace, which saves you time. I found a few of the rooms in the National Palace interesting, but the lines moved quite slowly.

If possible, I would spend the night in Sintra..then you have another morning to be first up at one of the two busy sites. Lodging is a better value in Sintra, too.