We loved Porto and in your shoes I would stay a night or two there if possible.

Plans are to spend 5 nights in Lisbon. I really wat to go to Porto, but we don't have time to spend the night there. We are considering renting a car, and driving to Porto for a day. Better yet, we would like to hire a tour from Lisbon to Porto and back. Anyone know of a company or guide that will do this ? Are my plans doable (reasonable) ?

Either give Porto at least two nights or visit Portugal another time. A day trip there is not a good idea from Lisbon.

I agree- you don't want to try to do Porto as a day trip from Lisbon. You need several nights there.

Lots of day trips you can do from Lisbon, and yes, the trains were very reliable when we were there in August.

If you want an idea of what we did on our trip to get your planning juices flowing, you can check out our travel blog at geekswander.com

Always happy to answer questions if it is helpful.

I have visited Cabo da Roca from day trip to Lisbon and its worth to visit if you have some time.

We are looking at a day trip to Sintra and Cabo da Roca. I read that it is quite beautiful. Also has some good seafood. Thanks

Okay, I am convinced we cannot reasonably do a day trip to Porto from Lisbon. Thanks to all for the input. However, we would like to do a day trip north of Lisbon. Can anyone recommend such a day trip from Lisbon ? We like history, food and wine.

Obidos? Our daughter and her DH stayed in the castle there and loved it.
https://www.travel-in-portugal.com/obidos

We took day-trips via train, from Lisbon to Cascais (twice) and to Sintra.

We loved the Hotel Avenida Palace in Lisbon.

thanks for the responses. We will be staying in Lisbon for 5 days, during which we will have a rental car for at least a couple of days. We plan to drive to Sintra one morning, see the sights, then drive to Cascais where we will likely have lunch. After we visit Boca de Inferno, we will drive back to Lisbon via Avenida Marginal, along the coast. Is this trying to do too much ? Just looking at the map, it lookd very doable. Thanks again for any input.

If you want a lovely view for lunch in Cascais, The Albatroz Hotel is an upscale place overlooking the beach: https://www.thealbatrozcollection.com/albatrozhotel
We thoroughly enjoyed our lunch there. There are charming little shops near the hotel, also.