Flying into Lisbon and out of Portoin late May. I am figuring three days in Lisbon and an additional day for a day trip to sintra. Then either rent a car or train to Porto and have at least two and a half days in Porto. The question is just enjoy the coast on the way up to Porto or train up there and rent a car and go up into the Douro Valley for 3 days?
Side question is should I spend an extra day when I am solo in San Sebastian or get on over to Lisbon. I will have two and a half days in San Sebastian but could stretch to three and a half and loose the extra day in Lisbon. I hate only having one month for vacation. I know this does not even cover the Algarve and Azores that'll be another trip
It sounds like you have your itinerary figured out. Spending a day in Sintra is a great idea. You can either join a guided tour or take a train from Rossio Station in Lisbon to Sintra (half hour ride each way).
I took Rede Expressos' bus (3 1/2 hour trip) from Lisbon's Sete Rios Bus Terminal to Porto. Like Claus, I would recommend traveling by train or bus. Here is the link to get bus info and tickets: https://www.rede-expressos.pt/
The Douro Valley was in my plan, but I didn't make it there because of the lack of time. Check out this article by The Telegraph, Why Portugal's Douro Valley should be on your holiday list, and you'll know why I regret not making time to visit.
Hope this info helps.