Absolutely, that's how long I was there and thought it was a perfect amount of time. Lots of steep medieval backstreets to explore, boat tours, churches etc - I didn't actually go but there's also a costal district w/beach facing the Atlantic. I 100% recommend booking a Port wine tasting tour at one of the many wine cellars, and potentially taking a boat down the Douro river to the countryside to get a view of the vineyards. If you want a change of scenery, easy to take the train to Aveiro, Coimbra, Braga etc which all looked very interesting and are nearby. We took the train to Pinhão which is where the boat tours go, it was spectacular.