Porto
Eat at Zenith, Taberna Santo Antonio (go a day ahead and make a reservation), and/or Bacalhau.
Drink port wine somewhere (either up the Douro valley or in one of the many warehouses on the southside of the Duoro river). I went to Ferreira.
Lots of great walking tours. I recommend doing one.
Porto Cathedral is one on the Camino Santiago, it's beautiful inside. You'll likely see some Camino folks with their seashells on their backpacks.
If you're a Harry Potter nerd, JK Rowling wrote the first book in Porto and the stairs at Hogwarts were inspired by the stairs at Livaria Lello (a bookstore). I like Harry Potter, but it's not worth standing in line for.
Lisbon
Sintra is a must. It's a great getaway from Lisbon. Castle of the Moors is incredible.
Time Out Market is a great food hall if you want to taste a bunch of different things from local shops.
Santini makes great ice cream.
You need to try Pasteis de Nata (which are little custard tarts). Everyone will tell you to go to the place in Belem, but I'm gonna recommend you go to Manteigaria instead. I think it's better and less crowded.
Terrapao makes a great sandwich.
Go party on Pink Street at night. Lisbon has a great nightlife scene and Pink Street felt like the hub.
LX Factory. It's an art center located about halfway between Belem and Lisbon center. It's got great restaurants, a cool bookstore, and some awesome local craftwork you can buy.