There are quite a few options here:
• Porto: Porto is a great city – especially if you like Port wine! You can easily fill 2-3 days with things to see and do. Some suggestions include visiting one of the many port-wine cellars and eating a calorific Francesinha.
• Povoa de Varzim: as mentioned above, this is a holiday resort with sandy beaches (as most Portuguese beaches in the north are rocky). Very picturesque, very nice. However, please note that the sea here is the Atlantic Ocean and, while pleasant enough temperature-wise in August/September, does not have the bath-like character of the Mediterranean. Therefore, larger waves and more wind should be expected.
• Algarve: Of course, the most famous holiday area of Portugal is in the very south. You can drive there from Porto in about 5 hours. Or there are options to go by train to Faro/Albufeira in approx. 7 hours.
• Lisbon: There are direct trains from Porto to Lisbon which take about 3 hours, similar to the time it takes to drive.