Advice for a Short Stay

Vacationing in Portugal in September for seven days, and will
be flying in and out of Lisbon . Have seen most of Lisbon before, so only staying there two nights, and doing a day trip to either Cascais or Sintra; then traveling by train to Porto and spending three nights there, including a one day river cruise/ winery tour to Douro. Spending last night of trip back in Lisbon, as don’t want to feel rushed getting to the airport the next day. Need ideas on where to spend the two days somewhere en route from Porto back to Lisbon that’s accessible by train and walkable. Would like a smaller town with relaxed atmosphere, but with enough to do ( love the ocean, but don’t want to just sit on a beach); prefer a scenic place that has historic or quaint architecture, nice cafes and restaurants, art galleries ,etc.) Any suggestions would really be appreciated. Thanks.