6 days in Portugal

I just returned from Lisbon a couple weeks ago and also visited Sintra. I consider myself a "blitzer" and do not usually go slow and/or explore all the museums or other options available. Partially because of my travel style and because I only had three days to work with, here is my take. I spent one day in Lisbon, one day in Belem and one day in Sintra. I was going fast and I barely saw the highlights. If I went slower, I could easily have spent another day or two in Lisbon/Belem and maybe even another day in Sintra (by the way the Pena palace was amazing). So now you are down to just filling up one more day and that should not be too hard. Hope you have a great trip!!

If you like architecture, Lisbon is an excellent place. Though a well-known destination, the Alfama -- the former salty sailors' quarter -- is a good start. Also, the Jeronimos Monastery with its Manueline style -- ornate Portuguese style -- in the Belem area should be on your list. When I went to Belem, I took the city bus. It went right under the stunning 25 de Abril bridge, which gives the Golden Gate Bridge a run for its money when it comes to beauty. Since you mention exploring, Rick's guide book offers a couple walks -- point-to-point walking tours with explanations of every important site along the way -- that average about 1 to 3 miles long. He has about 4 or 5 walks in there. I'd definitely pick up his Lisbon book. Look for the Gustav Eiffel-designed Santa Justa Lift in the center of Lisbon.

Tourists like to visit Coimbra, a famous college town. It's a couple hours from Lisbon Maybe consider that for exploring, though I didn't see it.

Take a look at Evora. Very easy by bus or train and a popular day trip. If you choose to spend a night there, we did, the Megaliths are amazing. I would recommend this tour. They will pick you up in Evora.

https://www.eboramegalithica.com/actividades?lang=en

And regarding Sintra, the Park and Palace of Monserrate. It doesn't get crowded and I personally think the palace is lovely, especially how the design blends into the amazing gardens. Pairs nicely with Quinta da Regaleira since there is a bus linking the two sites.
https://www.parquesdesintra.pt/en/parks-monuments/park-and-palace-of-monserrate/

Since you said "layover", I think you have no transportation worries. Lisbon is not "right next to" anything, even say, Madrid. But if you have a dodgy airline that might change the next flight on you, you might have no alternative but to wait for what they offer you. As a cruise port, Lisbon often has capacity problems with nonstops to major cities, for example.

I deny that I "like" to rent a car in Europe. But it can be hard to make the most of non-Lisbon Portugal without a car. While you may not like the idea, I want to mention that before we picked up our car, we took TWO full-day bus excursions from Lisbon that were very successful. We went to at least three different locations (including Evora) on each, and the trips were about 10 hours. We didn't feel regimented or pushed on to the next pottery shop (no pottery shops at all!), and had no transportation or navigation worries.

Just to make that clearer, unless you're a strong-faithed Catholic, how much "free time" do you really need at Fatima? We wanted to see that place, but 40 minutes free time, after the guide took us around, was plenty!

We did 4 nights in Porto and 5 nights in Lisbon. With only six nights, I would stay in Lisbon.

These ideas are wonderful, thank you! And I’m sure Peña palace is beautiful. I guess I just have flash backs to being trapped in the Vatican, looking upward at the ceilings and the art and wishing I could enjoy it, but just wanting to be outside and away from the crowds, and apparently those crowds weren’t even that bad.

Yes, I’ve got a layover in Lisbon through TAP. I know they get a bad rap pretty often, but I’m super excited about this opportunity to expand my horizons a little more.

I like TAP. I also did their stopover on the way to Italy. It was great. I’d skip Peña for this trip or just do the grounds. It’s crazy crowded and claustrophobic inducing. So many wonderful other palaces to see that I personally think are nicer.

We did 4 nights in Porto and 5 nights in Lisbon. With only six nights, I would stay in Lisbon.

I would agree. Stay in Lisbon and do day trips to Sintra and other areas if desired. There are lots to see and do in Lisbon, especially if you like walking around and poking here and there. You don't have to be a museum goer to find a lot to enjoy in Lisbon and outlying areas.

People do talk a lot about Pena Palace. My thoughts: worth visiting once but that's it. I wasn't totally wowed by it and I was a little turned off by the crowds, though it wasn't so much that it carried negative weight of significance. I would probably spend 2 days in Sintra as we walked everywhere and enjoyed it, and have a bit more to explore there.