5 days in Portugal in October - need itinerary advice please

We spent 9 nights in Lisbon and could have stayed longer. Two full days would give you a feel for Porto. Coimbra and Evora are very worth visiting.
CaiscaisI s a beach resort. There is plenty of seafood in Lisbon.

Another vote for Evora, once an ancient city in the Roman Empire. For local food, be sure to have Bacalhau a Bras, a salted cod dish, sometime in Portugal.

So Evora and/or Coimbra over Porto? In our limited time I think we have to pick one. Thanks

We never made it as far north as Porto (with a rental car), so that’s still on the agenda for some future trip. But Evora was a surprisingly great place, and dinner at the Pousada was wonderful. Before ever going to Ireland, we saw our first prehistoric Dolmen just outside Evora, which sparked an interest in that wide ranging culture from so long ago.

We didn’t have air conditioning in our room though, and one night in late June 2002 was uncomfortably hot. Maybe not an issue in October?

I wouldn’t travel to Porto for only one night. Save it for another trip. But, I would add my vote for Evora if you want to get out of Lisbon. It’s an easy train ride and would give you a feel for a smaller Portuguese town.
I don’t think five days is too long in Lisbon, though. Especially if you have been traveling for a couple of weeks by the time you get here - having long lunches on the miradouros, drinking wine, and enjoying the seafood might feel pretty great. Belem would fill most of one day, especially if you included the monastery and the coach museum. Cascais could also be a day, if you want a day by the sea. And, you can get good seafood anywhere in the Lisbon area.

We had 5 nights in Lisbon in April 2018, and we could have used an extra night. It all depends on what you want to see. We spent 3 days plus late afternoon/early evening on arrival day sightseeing in Lisbon. Then we took a daytrip to Sintra, which included a very brief stop in Cascais. We liked what little we saw of Cascais, and it would have been nice to spend a day there. So I don't think 6 nights in Lisbon is too much, including a day trip to Sintra and a day trip to Cascais. We didn't have time to see the Tile Museum, and it would have been nice to have more time for leisurely wandering through Lisbon and relaxing with drinks at the miradouros.

Do you like art? We spent a lot of time at the Gulbenkian Museum. It is fabulous! We spent a whole day touring the Sao Jorge Castle and wandering through the Alfama. Seeing the sights in Belem takes almost a whole day. And there were some smaller churches we visited, too. You might be tired during your 3rd week of traveling and would need a relaxing visit in Lisbon.

Porto is wonderful, but with only five days, I don't recommend wasting time on travel. You need three full days in Lisbon, but:

Do day trips to Evora, Sintra, Cascais, Obidos, Batilaha Cathedral and Navarre.

Thank you for all of the good advice. We’ve decided to rent a car during our Lisbon stay, and use it to explore the local region instead. That will give us the most flexibility and allow us to travel and explore at a more relaxed pace. We traveled that way in San Sebastián and it was one of our favorite trips. Can’t wait!