We did a Portugal trip about 4 years ago with one week in Lisbon/Sintra and one week near Porto. We rented a car and it was an easy drive to Porto and back to Lisbon. Highway was pretty empty both ways. I love Sintra and would definitely add it to your trip. Nice thing about having a car in Porto is that there are amazing small towns and villages in that area if you plan to explore outside of Porto.

I'd say definitely rent a car to drive to Porto then take a train quickly back to Lisbon.

Cascais and Sintra are basically in Lisbon - few minutes by train so it would make little sense to stay there to break your journey.

But Obidos is one fantastic place to stay or stop by - one of the finest walled cities I've seen in Europe - really ancient looking - check out the Pousada (?) there - the Portuguese equivalent of Spain's famed Paradoes - this one in an ancient edifice built partly into the old walls.

Coimbra would make a great stop as well.

And you may consider Nazarre, an old fishing town where folks still fry fish in alleyways to sell - great base for visiting the famous Batalha Monastery and the one at Alcbaca as well - easy to stop by these with a car en route to Porto.

Porto also offers some great day trips by car - like Braga and the Duoro River Valley.

I've thought about the Douro Valley, but it's late March, so not counting on the weather to be nice enough for that. We won't venture north of Porto, however much we'd like to, since we don't have much time, and have "done" the north previously. Any particular stops along the Douro?

Nazarre and Obidos are good recommendations--I'll check them out.

As for car/train--I was thinking we'd just keep the car until we get back to Lisbon--David's conference may be a little out of town, trying to find that out. I know a car would be a pain in Porto itself, but very handy elsewhere.

I did a trip along the Duoro in November and the weather was fine - we did it by train and stopped at a nice town but cannot recall its name - I would not say this was a must to do, in retrospect but very nice.

Obidos would make a nicer stop going back to Lisbon as it is way over half the way there.

Fatima if interested in pilgrimage sites would be a detour off the straight line some would like that - I have never been so cannot comment.

I've thought about the train, but a bit reluctant with all the cutbacks, strikes, etc. I guess I should look into it.

I stayed in Coimbra and found seeing the library one of the highlights of my stay in Portugal, but otherwise would not recommend spending a night in Coimbra. I believe that even if you take the train, you could made a stop in Coimbra and with Tuk Tuk tours to put your luggage in their offices (or the train station) and then be taken up to the library and back to the train again, probably with a short whirl around town to get the gist, but it is the university library that is the world class unique sight.

I really enjoyed the six days I spent in Lisbon before moving on to Porto, so I am tempted to say skip the car tour and just divide your time between Lisbon and Porto, enjoying each, taking the train between.

If you decide to drive, and you will only be in Porto two nights, consider staying at the Yeatman, where you can easily park, and have the views, the great food and the port wine, plus go across the river to Porto proper and have all that too. A "splurge" but relative to what such a stay for 2 nights would cost elsewhere in Europe, it would tempt me.

Were I driving between Porto and Lisbon, I might be tempted to stop at Nazare to see, if in March, I could see some of the world's biggest waves.

Looking at the map of Obidos--looks like there are two areas to stay, Obidos itself and Caldas da Rainha. Looks perhaps like a resort area? How important is it to stay right in Obidos? (We like to be able to walk around town.) Is the Pousada in a walkable area?

Guess what I'm saying is it's tough to see what's rural and what's central in Obidos.

Obidos is hilly and compact. The Pousada is lovely but there are lots of accom. in the immediate area. Check out Pousadas.pt as I got an email yesterday telling about great savings but I don't remember the dates available..Caldas da Rainha is larger and not as picturesque if I remember correctly..BTW when we were there we had a car and pousada package which was outstanding..Maybe Pousadas.pt still have these specials..Check Pestana Group also..Yeatman is awesome but dear inthe $ area..but you get what you pay for.

A few notes:
1 - In central region of Portugal I'd chose romantic Palace Hotel of Bussaco (Buçaco in Portuguese sp). It has a very relaxing forest, hard to believe nowadays that is was once a battle field during French invasons, back in 19th century.
2 - The historical walled town is Obidos. Caldas da Rainha is a recent working class town, without any special attraction as a tourist destination.
3 - Everything in Obidos is walkable. Obidos has a main street, with a gate in one end, a square with a main church in the middle and a castle in the other end, the whole circled by high walls.
4 - Pousada de Obidos is a real castle. It has 3 or 4 rooms, they charge high prices and it's difficult to book. The logical alternative is Estalagem do Convento Obidos. I think it offers more by less.

Thanks for the information. Would Obidos be worth two nights, or not? The drive from Lisbon to Porto is a fairly easy day, so would an afternoon in Obidos be enough, or would spending another night there be worth it? Are

Probably won't stay in the castle--in general don't like isolated hotels, since we like to walk around town. Trying to get the dates sorted out so we can book hotels.

One night is ok, 2 nights are absolutely too much. As I said before, Obidos has 1 main street, one castle in one end and a gate in the other. To walk around one afternoon is ok, however a night there can make sense. Obidos can be somehow crowded during the day. When the hordes take the buses and the night falls, it gets a different atmosphere. There are places along the main street where you can have a light meal like a chouriço assado (grilled sausage) with bread and dring a glass of red and these places work mostly after dark.
IMHO Obidos doesn't sound as authentic as the historical border villages of the Alentejo (Marvão/Castelo de Vide and Monsaraz) however it has the benefit of an advantageous location near Lisbon and the main highways leading north.

Thanks--I guess we could just spend time there en route if we wanted. Still working on dates, trying to decide how many nights to spend in Porto (been there before.)

Is driving to Porto and keeping the car while there going to be a royal pain? I'm thinking we could take a train to Porto. Then rent a car to drive back to Lisbon, making stops on the way. That way we can dump the car as soon as we return to Lisbon, but perhaps stop in Sintra or Cascais on the way into town.