HELP! 2 day driving itinerary from Porto to Lisbon.

Coimbra and Evora. Sintra is an easy train ride from Lisbon as a day trip.

Not sure it's worth the trouble of renting a car for only two days between Porto and Lisbon. If you had a few extra days, it would make a lot of sense. With only two days, I'd just take the train or bus between cities and either do day trips or maybe a stop in between (e.g. Coimbra). As said above, the direct train from Lisbon to Sintra and back is super easy. Still, I hear driving in Portugal is not difficult.

...but if I had a car, personally I'd probably stop for a night in Nazare - spent only a few hours there but loved it, would loved to have spent a night. Coimbra is worth half a day at most, to see the university. (I spent a few nights there.)

I concur with others who recommend Coimbra, Evora, and the lovely fishing village of Nazare. Consider stopping in Nazare for lunch. We spent the afternoon and had an enjoyable time. We spent 2 nights in Coimbra and one night in Evora on the RS Portugal Tour. We especially enjoyed our tour of the University in Coimbra. It was September and the students were there for orientation. Evora has the Roman influence that includes the Temple de Diane & Aqueduct. One of the more interesting sights was the Capela dos Ossos (the bone chapel). We also spent a delightful half day tour of Sintra before the RS Tour.
Enjoy your trip!

There are a multitude of interesting places that you could visit (sorry, Nazare wouldn’t be on my list). If you want traditional towns and churches/monasteries, then towns such as Tomar, Alcobaca, Aveiro, Batalha and Evora would be worthwhile. Sleep in Evora or Tomar, which are both interesting in the evening.

Check out the Portuguese Pousadas. There is a great one at Obidos.
Nazarre and Bathila Cathedral are great.
http://www.getty.edu/art/collection/objects/200408/francis-frith-bathala-cathedral-portugal-from-the-south-english-about-1850-1865/