Portugal in 6 days

Our trip to Portugal was just over 20 years ago, but also a short trip. We split our stay between Lisbon and Cascais, and were happy with that decision. We did a day trip to Sintra and loved it!

We did Portugal 2 years ago and loved it. Spent two weeks doing Lisbon, Porto and Douro Valley. My recommendation for 6 days.
-Lisbon. Lots to do and food is great
-Porto, not that exciting but again good food. If only six days , stay in Lisbon area.
-Douro valley is beautiful but I've done enough wine tours in my life. Same wines in town/
-Take 1 overnight to Nazare area. You can rent a taxi and do door to door
-Sintra avoid. Even Rick steve's tour guides will tell you that. We did it anyway, waste of time and money.
-Took day cruise up the river from Lisbon. Enjoyable
-Nautical museum. One of the best museums I've been to.

Any adventure in Portugal is time well spent.
Is it worth staying in one location? If you do, you will miss Porto - although the train ride from Lisbon is a mere three hour journey. I would include Evora as a "day trip" - there are many travel companies that off this for around $75/person. Sintra probably will be crowded, but the Moorish Castle is a pleasant walking experience. Be aware that walking in Lisbon is either uphill or downhill - sometimes a little steep. If you elect to focus your time in Lisbon, a rental car will be more of a liability than an asset.

Like others up thread, we loved Portugal. We took the RS Portugal Tour in 2017. We arrived a couple days before the tour began. One of those days we hired Lisbon Riders for a small group tour to Sintra, Cascais and other stops along the way. It was a great introduction to Portugal. It was one of our favorite areas we visited. Our guide was very knowledgeable.

Due to your limited stay, I would recommend basing yourselves in Lisbon and doing a couple of day trips. Their is plenty to see in Lisbon. We also enjoyed visiting their Aquarium prior to our tour.

Stay in one area and my choice would be Lisbon. You don’t need a car as the trains (and metro) are great in this area. From Lisbon you can visit Sintra (doubt if full crowds will be back by December), Cascais, and even Evora. But, with only six days, I doubt you will have time.

There are a lot of outdoor opportunities in this area but be aware that December is the rainy season so you will likely have a bit of rain - or even a lot. If you want to focus on outdoor activities you might want to consider here in Cascais. December is never a big tourist time and the walks along the sea are great. You can day trip easily to Lisbon and Sintra.

Having been to Portugal twice (2016 and 2019) and loved it both times, I concur that with only 6 days you should stay in Lisbon. Porto is nice, but not as nice as Lisbon and the trip to and fro will eat into your time. Sintra, whilepritty, is crowded and much too touristy (pre COVID of course).

Be sure and take the water taxi over to Cacilhis for lunch at Restaurant Farol.

I agree with the people who suggest staying in one place since you only have 6 days. And I agree you should stay in Lisbon. It's a wonderful small capital city that is so different from the huge European capitals. We loved it! It is inexpensive and the food is delicious.
We had 5 nights in Portugal, and we spent all 5 nights in Lisbon.

We did take a guided tour by bus to Sintra, and we really enjoyed it. We went in April. There were crowds, of course, but I didn't think it was unbearable. I doubt the crowds will be awful in December, and especially because of COVID. We visited Pena Palace, so colorful and fun to walk around and take photos. The inside is mediocre but the outside is so funky. And we walked through a very pretty park to get back down to our bus. There are quite a few other palaces/castles in Sintra, but we only saw Pena Palace. Then had lunch sitting outside in the center. We then went to Cascais and enjoyed seeing the coast. Made a short stop at Boca do Inferno. We normally don't take bus tours, but this was the end of a 3-week trip; we were getting tired and decided to be lazy. But you can easily do a day trip to Sintra by train.

For only six days, stay in Lisbon.

Also, Evora, east of the city can be done on a day trip.
Cascais is nice, also the ancient town of Obidos that has the original Roman wall. Sintra is touristy, but it is still worth the trip. Don't try driving there, since parking is difficult.

For your next trip to Portugal consider a Douro River cruise. We did a 7 day cruise and it was wonderful. Also, in Porto, stay in the In Patio Guesthouse, best B&B ever, anywhere.

We spent nine nights in Lisbon before exploring the rest of the country. There are many day trips you can take from there. With only six days( seven nights?) stay in Lisbon.

thank you all
We are thinking we will stay in an apartment in Lisbon and take the day trips
Can anyone recommend a private tour company that they trust for tours?

There are tour companies and private guides listed in Rick Steve's Portugal book. That will give you a good start.
Also, although others suggest avoiding Sintra, I would add that we stayed in Sintra for three nights at the start of a two week trip to Portugal and really enjoyed it. The day-tripper crowds were bad during the day, but not particularly bothersome when staying there and visiting the popular sites early in the morning or late afternoon. However, do not take a car to Sintra as parking is at a premium and regulated heavily. We took at taxi (arranged by our B&B) direct from Lisbon airport to Sintra at start of trip.
If you only have 6 days, though, probably better to stay in Lisbon and day trip or stay in one other city, at most.