How long will you stay in Lisbon? It's a wonderful city, and so is prto.
If you are taying a bit longer, make a day trip to Sintra which has a plethora of beautiful sights. If you want to go to the beach, you could go to something like Cascais or Oeiras. Keep in mind that though it's very hot outside, the water is ice ice cold (well for me it is, but I am half brazilian haha).