Porto is a colorful city that has become an instant favorite. Although its core area is fairly compact, it is not for weak walkers: the city is built into hillsides, you cannot go more than a couple of minutes without ascending or descending a non-trivial slope or staircase. For those who can take it, the city is very walkable: we did not use any public transport aside from the Gaia cable car for our entire stay there.

Most of the key points of interest in town can be visited in a loose loop (the starting/ending point can be adjusted to your liking). Let's say we start at the incredibly tiled rail station of São Bento. Next stop is the Cathedral, with a possible detour to take a closer look at the façade of Santo Ildefonso and also at the São João National Theater. The church of Santa Clara is also around the corner here. Next, cross the bridge of Luís I on the upper level; the views are amazing. The monastery of Serra do Pilar is at the end of the bridge on Gaia side. The best way to descend to Gaia waterfront from here is by taking the cable car (comparatively expensive but mostly worth the cost). Walk along the quay back to the bridge; some of the most popular wine caves are here, if you'd like to visit.

Next, cross the bridge back on the lower level and enjoy the Ribeira colorful and bustling riverside district. Afterwards, go to the Jardim do Infante Dom Henrique. Make sure to visit both the Palácio da Bolsa (guided tours only on the timed-entry basis, so you may have to buy tickets at one point and return for the tour later in the day; the palace is impressive enough to make that a negligible inconvenience) and the church of São Francisco (you have to pay for a combined church-and-museum ticket here, but it is worth it for the church alone).

After that, walk up towards Cadeia de Relação; depending on which streets you take, there will be other churches and minor picturesque points along the way. Practically next to the former prison that is now a center for photography is the church of Clérigos, whose elegant tower rises above almost everything in town and therefore is a popular attraction; the effort and expense of getting to its top platform is not for everyone, but avid tower-climbers will not want to miss it.

A block away from the church is the wide expanse of Praça da Liberdade and Avenue dos Aliados, the "center" of Porto. And with that, you completed the loop, since you are now again within shouting distance from São Bento.

There are obviously other points of interest that require detours (e.g., Casa da Música - not really within walking distance, it would require the use of public transport to get to it) and other notable squares and buildings both in and around the town center. You can leave them for the second day. However compact, Porto cannot be fully experienced in a single day. At least two full days are needed to see it without rushing.