This is definitely a city to come back to. I think that immediately after London, Porto is one of my favorite cities in Europe.
The public transportation is a bit more cluttered than the one is Lisbon. In Porto, they have 5 subway lines that are great for reaching anywhere in the city and outside of it. It's based on zones, so you will have to pay different ticket price to reach a certain station. It's pretty expensive (starts at €1.20 and can go up to €5.15), but I think it's worth the money if you have few days to spend in this city and you want to move fast. Take note that the initial rechargeable ticket costs €0.60.
I stayed in the center of the city, few steps away from the library where the famous J.K. Rowling wrote Harry Potter. This library has an entry price of €4 if you are interested checking it out. I did not, because I found that pretty expensive.
In the first morning I drank my coffee at the oldest coffee place in town: Majestic. Rumors say that J.K. Rowling also wrote on the parts of the book on napkins from this place while enjoying her coffee. The coffee price starts at €5 and I'm sure you can find better and cheaper ones around the city. It has nothing special but the place looks fancy enough.
The most spectacular place in Porto is Dom Luis bridge. You get a really nice view of the city and if you go there when the sun sets, you will get a lot of beautiful shots.
I really enjoyed Porto and I think I'll come back here someday.
Thanks for reading out and I hope you found out some interesting things.