This old neighborhood is a UNESCO World Heritage Site and is filled with cute cafes. I recommend a walk on the upper level of the Dom Luís I Bridge to see amazing views of Porto.
Across the bridge is Vila Nova de Gaia. It is just a short walk to the Jardim do Morro or fortress-like Monastery of Serra do Pilar, which has the best views of the Douro River.
The Casa de Serralves is a great place to see contemporary art. It is an Art Deco villa part of the Serralves Museum. And a tour of the Palacio da Bolsa is worth it!
The Baroque-and-Gothic Church of Saint Francis is one you'll want to enter to truly appreciate. The interior has golden gilt woodwork. Another option is the Capela Das Almas.
Book Store
One spot that I was determined to visit in Porto was Livraria Lello, one of the oldest bookstores in Portugal. The bookshop dates back to the late 1800s.
Visit the São Bento railway station even if you're not taking any trains to/from Porto, it's worth popping into the Sao Bento station to admire its iconic blue and white entryway.
Other cool architectural spots worth mentioning in Porto are the Cafe Majestic, which is an Art Nouveau cafe and the Casa da Música, which is home to the national orchestra.