- Somewhere between 30 and 90 minutes away from Porto by train;
- Somewhere a little bit cultural, maybe quirky, traditional;
- Something that is away from the hustle and bustle of a big city;
- A few little old bars / cafes are an essentiality;
- It's for a group of old boys.
I would assume anything within Porto, Vila Nova de Gaia and Matosinhos are out of the question. Maybe even the likes of Maia (Parish) and Ermesinde (Parish)?
It really depends how big and busy you would consider a city to be. In Portugal places that are considered cities would still be towns in the UK.
I think Braga is also quite big and busy, at least as much as Matosinhos or Vila Nova de Gaia.
Guimarães is a tad smaller and, being the birthplace of Portugal, it's a must go in cultural terms.
Aveiro is even smaller in comparison, and most stuff is less than 30min by foot, including the University.
All destinations in both the Underground (Metro) and Overground (Urbano) services are 60min or less from Porto-Campanhã.
Would rail related things be essential within that day out? A day out for railfaning is quite different from a day out for tourism/leisure.