Porto's surroundings are worth at least a day or two of exploration.
The Douro Valley rail route is one of the finest train journeys in Portugal. The line joins the river about 60 km inland and then follows its narrow, winding valley across the country, through terraced vineyards and dramatic gorges. No destination required — the ride itself is the point.
On a more relaxed note, the Atlantic coast promenade along Avenida Brasil makes for a pleasant half-day outing. Locals head here on Sundays to walk, sit at seafront cafes and eat sweets from the street sellers. Bus 1 or the historic tram 18 will get you there.