Sintra: Overnight stay, or day trip from Lisbon?

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In my opinion, you need the time in Lisbon. So much to see and do there. If you really want to carve out time, just do Sintra as a day trip. And by the way, I wasn’t impressed with Cascais. Enjoy Portugal! One of our favorite trips!

Three nights in Lisbon equate to 2.5 days. If you take one day for Sintra you have only 1.5 days for Lisbon. In my opinion that's not enough time but it's your trip. Do some research and make a list of what you want to see and do in Lisbon. There are many worthwhile sights and neighborhoods. Organize your list and figure out how much time it will fill. Then you can decide how many nights to spend in Lisbon.

It's always a hard decision: how to spend limited time when there's so much to see and do. Good luck!

I absolutely love Sintra and have been there 3 times. But for the amount of days you have in Lisbon, I would do it as a day trip. There's not enough time to see Sintra and Cascais/Estoril in one day. There's not enough time in a day to see much of Sintra.

Regarding gardens, take a look at Monserrate, and Quinta da Regaleira is fun with their caves etc. Pena is the palace that a majority of individuals go to. It's interesting to look at from the outside, but I very much preferred the other two. I love the outdoors and gardens.

https://www.parquesdesintra.pt/en/parks-monuments/park-and-palace-of-monserrate/history/

As a side note, a rental car can be a hassle when visiting Sintra as a day trip and best to take the train. If you do drive, leave early and park it in a lot outside of town.

And I suppose, you could decide to skip Lisbon and just stay in the Sintra-Cascais area. I understand there is some hiking in the area that a car would be helpful to get to. I've thought about spending some time in that area myself for a future trip, but then, I've also spent 7 days in Lisbon on my last trip.

Thank you so much for the insight! I'm quite torn at how to approach this, as I find myself attracted to what seems a more nature/outdoor focused experience in Sintra, but there's a long list of sights in Lisbon that we'd love to visit. Looking closer at our itinerary, would the following be too aggressive:

Day 1: Depart Porto (early morning) via rental car, with stops in Nazare (2 hours) and Obidos (2 hours). Arrive early evening in Sintra.

Day 2: Explore Sintra, specifically Pena Palace, Monserrate and Quinta da Regaleira. Stay in Sintra overnight.

Day 3: Depart Sintra (early morning) to Lisbon

Day 4: Lisbon; walking tour (any recommendations??)

Day 5: Lisbon; food tour (any recommendations??)

Day 6: Depart Lisbon (early morning) for Algarve

This plan would give us roughly 2.5 days in Lisbon.

I have always visited Sintra as a day trip. Earlier in the day is better. It is a unique and beautiful location.
A different day trip from Lisbon is Evora ,about 1.5 hours by train and is well worth the trip.
Évora, World Heritage Site | www.visitportugal.com

Since you're driving from Porto to Lisbon, take a look at some of the towns/beaches along the way (Silver Coast). Nazare gets a lot of tourists. And since you have a car, have you considered the monasteries, Batalha or Alcobaça? I liked Obidos, but liked the monasteries much more.

All 3 palaces in a day are doable, but can be exhausting. If it ends up being a long day, perhaps you can visit one of the palaces in the morning before leaving for Lisbon.

Edit to add: For Sintra, the only ticket that is timed is if you want to go inside Pena Palace. Otherwise Park of Pena (gardens, all of the outside including turrets) and the other tickets can be used within a year. This is to say, you have some flexibility and I would recommend enjoying yourself at each palace and don't feel the need to rush to the next one.