Porto is an easy base for day trips by train. Aveiro is the relaxed one with canals, sweets, and a windy coastal finish. Guimarães is the full leg day. Braga is the balanced option: calm centre first, then Bom Jesus as the big finish.
We did these without hiring a car, which kept the days simple. The pattern was usually the same: walk to São Bento, buy tickets by card, check times, accept that the day will involve more steps than planned, then come back to Gaia tired and hungry.
How we would choose
For the lowest-effort day, choose Aveiro. It gives you the most obvious contrast from Porto without asking too much from your legs.
For the most memorable physical day, choose Guimarães and walk to Penha. It is the kind of day that feels funny afterwards because the effort becomes half the story.
For the most balanced day, choose Braga. The centre is pleasant without needing a strict itinerary, and Bom Jesus gives the day a proper final act.