My wife and I have four days to plan following conclusion of our late October 2025 "Best of Portugal" tour. As the tour ends with two full days in Porto, and we fly out of Porto, we could just spend the extra days exploring the city more deeply and thoroughly. Yet I'm wondering whether spending six consecutive days there may be too many. We typically really enjoy university cities/towns so we are also considering returning to Coimbra for the extra days. Any thoughts or alternative suggestions?
With 4 additional days, I would give 1-2 to Coimbra and the remainder to Porto before your flight home.
Especially since you enjoy university towns, Coimbra would be a lovely place to relax and enjoy the botanic gardens and a stroll through the park that runs along the river. And of course, visit the University and perhaps the Machado Museum.
I would want to be back in Porto at least the night before my flight, and perhaps 2 nights, depending on exactly how many you have. You could give Coimbra 1 night, timing trains to maximize your time there. Or take a later train to Coimbra and give it 2 nights, arriving earlier back in Porto for some additional sightseeing.
My partner and I spent 4 nights in Porto and 2 in Coimbra in Aug 2023. Our trip report might be useful reference: https://community.ricksteves.com/travel-forum/trip-reports/trip-report-portugal-aug-sep-porto-coimbra-lisbon-then-to-spain
Looking at your itinerary, you will get a good overview of Porto on your tour. An extra full day or two to explore more on your own is nice. You could visit the port wine lodges in Vila Nova de Gaia. Also, I love the Foz do Douro area. It's where the Douro meets the open ocean. If you venture out there, play a round of mini-golf. My husband and I have done it on two different visits.
For your other extra days, venture out of the city. Porto has some great DIY day trip options. All the ones I mention can be easily reached by train, and the tickets are very reasonable. The towns themselves are walkable.
Guimarães has a beautiful historic center and multiple churches. The Museu de Alberto Sampaio offers great background on the city and even has armor worn by the king during a medieval era battle. You can take the Teleférico de Guimarães up to Mt. Penha. It offers nice views and a little bit of hiking. There are plenty of picnic tables if you want to bring lunch along with you. If you have a sweet tooth, try the traditional dessert Tortas de Guimarães.
Aveiro has strong ties to the fishing industry and offers canal tours in traditional boats. It's also pleasant to just walk along the canals. There are a number of museums. The very popular ovos moles dessert, which you will see throughout Portugal, originated in Aveiro. When I visited in 2023, there was a big construction project going on in the city center. I would check the status of this before visiting.
Braga is filled with churches and Roman ruins. When we were there, we didn't have enough time to visit them all in a single day. The Museum of Archeology D. Diogo de Sousa has a cafeteria that offered a fantastic cheap lunch, and it was filled with locals. (On that note, I've also had lunch at a college cafeteria in Coimbra. Another great experience!)