We are talking about a wow list. You can photograph old people on the streets. You can focus on colours because Porto is full of tiles and warm paint. You can compose on old stone, doors and windows, all with nice textures.
The skyline is a must-photograph and you may realize that what you need is a tele-photo. The old bridge that connects Porto and Gaia is a composition challenge.
Early morning light can be exceptional, but if we find ourselves surrounded by fog, we can plan some moody photos, perhaps in b&w. All the layers of little houses can give incredible tele-photo composing.
Late afternoon warms the colours and could be the ideal time to go to Gaia and try to combine the river, the boats and the skyline. Sunset will depend on your luck.
Night photography will be mostly around the river side, composing from Gaia to Ribeira and vice-versa. We may have time and stamina to try some night photography around Aliados and São Bento. By the way, the train station is a place not to miss.
Check PINTEREST and also INSTAGRAM to see all the potential.