Please review Itinerary

Thank you! I thought it would be easier to travel by car, but if the cities are easily accessible by train I will definitely consider this.

Hi
Public transit is fairly good in the large cities and there are good intercity bus and train connections, however it's not as flexible as a car. Carrying luggage around every few days, struggling for space in the commuter trains, etc, makes public transit less appealing.
Car rental gives you more flexibility but depending on the time of day you may be stuck in traffic, looking for parking, trying to not get lost.
It's a tough choice when you plan to move to several bases - I usually use public transit if staying in one or two base cities and a car if using more than two base cities.

Lisbon - Sintra - Porto - Lisbon is ok but I would stay in Lisbon and Porto only and use them as bases for day trips, makes more sense if you're using public transit.

This would be my itenirary:
Lisbon day trips -Sintra - Cascais
Porto day trips - Braga - Guimaraes - Douro Valley cruise.

If with a car and time permitting I would visit some of these on the northbound and southbound trips:
-Conimbriga Roman ruins
-Coimbra city
-Batalha monastery
-Alcobaca church
-Aveiro city

Happy travels

With only eight nights, three moves is too many, IMO. Stick with Lisbon and Porto and day trip to Sintra. I’m guessing the crowds will be back in force in Sintra by that time so a small group tour from Lisbon might be your best bet. They would drive you from site to site so you don’t have to rely on public transport within the city.

Don’t try to cram too many day trips into your Porto stay. There are many companies offering small group tours to the Douro, which I highly recommend. Matosinhos is a bus or metro ride from Porto and is a great place for a grilled fish lunch. And, the Port lodges in Gaia are a great afternoon of port and wine tasting.

I would recommend taking the train between Lisbon and Porto. They are inexpensive, especially if you book a couple of weeks ahead at cp.pt

Travelling by car is almost always the worst option when visiting cities in Europe, take the train.

Hi!

You wrote "We will be renting a car and want to see Lisbon, Sintra and Porto". In my opinion, you're not renting a car, you're renting a big headache! I live in Matosinhos and work in Porto, downtown. Last week I had a few things to do after work, and for some stupid reason I decided to drive to the city center. Although I know the best streets, the worst traffic hours, the best places to park, etc., I felt the most stupid person in the world! It was a nightmare!

Concerning your trip, I absolutely agree with everything Kathrying said . Her suggestion is the most valid, in my opinion.

Helena

Thank's for your advise we have decided to not rent a car and take the train. We will also be dividing our time between Porto and Lisbon.

The best way to get to Sintra from the Lisbon airport is to take either Uber or taxi. I learned this the hard way. I took the metro to Oriente station in Lisbon (easy), then wasted at least 1/2 hour finding the ticket booth for the Sintra trains. Then I took a taxi to my hotel. Even as a solo traveler, Uber would have been affordable and saved me a couple hours of stress and travel time. (my visit was May/June 2019 and taxis were only a little more expensive than Uber). For 4 people it's a no-brainer. Then taxi/Uber to Oriente station for the train to Porto.

This is the website for train info and tickets. You must use the Portuguese names. You will want Lisboa-Oriente and Porto-Campanha. The Porto station is not in the center of town, take a taxi to your lodging. The AP trains are the newest, fastest and most comfortable. The IC trains are fine. Avoid the others. There's a discount for older riders, but I don't remember from what age.

When you return to Lisbon, it may be faster to take the metro from Oriente to your hotel. Lisbon traffic can be excruciatingly slow at certain times of the day. If you have a lot of luggage, be aware that Oriente is not friendly. The train platforms are on the top level. There are escalators and small elevators down to the next floor where tickets are sold. Then it's 1 or 2 (I remember 2, but I could be mistaken) to the ground floor and there's only one elevator that's not easy to find. From the ground floor there are escalators to the metro. The metro is modern and usually easy to manage with luggage.

Thank you so much! This information is very helpful! I definitely would want to pick the newer trains, I once had a bad experience in Italy on an old train. Definitely don't want that to happen again.