Skip-the-line / pre-booked at attractions in Lisbon, Sintra

You can purchase tickets online for Pena Palace in Sintra - and the National Palace and Montserrat. The one with the biggest lines is definitely Pena and I highly recommend purchasing them ahead of time.
And , you are correct, Sintra is always crowded during the day with Pena being the biggest draw. Getting there early and not on Monday (when Lisbon museums are closed) is your best bet.

Jerónimos monastery also has very long lines so purchasing tickets ahead is best. If you have any interest in climbing the Belem tower, it also has very long lines in the summer.

There are not timed tickets as at The Alhambra, etc. If you have the option, I recommend not coming during the summer months, as the crowds have increased so rapidly, Portugal is lagging a bit behind (IMO) in developing the best strategies for managing the crowds.

Thank you Kathrynj,

Good tip on not visiting Pena Palace on a Monday! Crowds there look scary :)) it's too bad there are no specific time slots that you can buy tickets for as the number of visitors is not limited for Pena Palace. I guess it's a blessing and a curse at the same time. You can always get a ticket for any of the days but that means it can be overrun by crowds. I think I enjoyed having a particular time assigned for Alhambra in Granada and the Alcazar in Seville (even though scoring tickets was pretty stressful!).

Still haven't found any information where and how to buy tickets for Mosteiro dos Jerónimos. All I see on their website is "Ticket sales: at the National Museum of Archeology".

I apologize for my mis-step, as I can’t find a place to purchase Jerónimos tickets online either. Guess I dreamed that!
However, I found a couple of other options. If you go to the Belem Tower, you can buy a joint ticket for both. And, even better, the national archeological museum (right next door to the monastery) has ticket machines in the lobby where you can purchase tickets. If it’s summer when you are here, I recommend the second option as the lines at the tower seem really long.