All in all, if you based yourself in Lisbon and find yourself with an extra day, I would absolutely recommend a day trip to Porto. Yes, the travel time of 3 hours each way may seem long, but you can always sleep, read, or work and since there are early and late departures, you can have a full 8 to 9 hours in Porto.

It is absolutely worth visiting Porto from Lisbon, provided you have at least two full days to spare. While you can do it as a day trip via the high-speed "Alfa Pendular" train (which takes about 2.5 to 3 hours each way), a single day barely allows you to scratch the surface of Porto's cozier, moodier charm. Porto offers a completely different vibe than Lisbon; it feels more ancient, intimate, and "granite-hewn." Highlights include the Ribeira district (a UNESCO World Heritage site), the stunning blue-tiled churches like Capela das Almas, and the world-famous Port wine cellars in Vila Nova de Gaia. If you stay overnight, you can experience the city's magical evening atmosphere along the Douro River and visit the iconic Lello Bookstore without the peak mid-day crowds. For travelers who have already seen Lisbon's "grandeur," Porto provides a more traditional and romantic perspective of Portugal that many visitors actually end up preferring over the capital.