Oh the things you'll do & see in Porto! We saw Porto for this first time this year, and fell in love this beautiful, vibrant city. Sadly we only had 3 nights there, and that was absolutely not enough time, as far as we were concerned. The next visit will definitely be much longer!
There are a few must see sights in the city, as with any place you visit, but our favorite times here were when we just gave ourselves the freedom to explore. Nothing beats strolling along the streets and lanes and just letting yourself take it all in. It's so wonderful when you turn a corner and encounter a breathtaking view, a quaint cafe, or other local points of interest not listed in the guidebooks or reviewed on Tripadvisor. It truly makes your visit feel more enriched, and you won't regret having taken the road less travelled.
That being said, make sure you visit those other touristy areas too, because they are popular for a reason. We had limited time, but while we were there we made sure we visited places such as The Lello Bookstore (for it's beautiful staircase), Clérigos Tower (great views of the city!), The Chapel of Souls, Luís I Bridge, São Bento Station, Avenida dos Aliados, Liberdade Square, Cais da Ribeira, the Gaia side of the Douro River for tours of port wine cellars and beautiful views of Porto, and The Portuguese Center of Photography (a must for us).
There were many other great spots that we missed out on, due to time constraints, but we'll make sure to check those off our list next time. There are gardens, a palace, monuments, art galleries, so many bridges and museums, churches, and markets such as the Bolhão Market (which was closed while we were there). There is really no shortage of attractions, so make sure you give yourself plenty of time to explore them. We thought 3 nights would suffice, but that wasn't the case. I'd recommend at least 5 to 7 nights to fully immerse yourself in all that Porto has to offer. 
Until next time, make sure you are following me on Instagram and Twitter and remember to be kind, be loving, & be better!