I don't know the area all that well but Matosinhos is about 6 miles from the airport and has a rather large sandy beach. It is also on the tram/metro line so you can easily get into the city of Porto if you want to. There is though a large commercial port and oil refinery in the area, which might spoil your views!
There's also Espinho across the river to the south, which seemed a popular area with city dwellers when last I was there. However that's a lot longer away, about 23 miles, but my route finder suggests it's only about 35 minutes by road. Rather longer (an hour or so) by metro and train.
Other places that I really don't know much about are to the north of the airport, such as Vila do Conde and Povoa de Varzim. You will need to do a bit of digging to find out what these offer holidaymakers. They obviously cater for people on day trips but what there might be in the way of hotels etc I really don't know.
Hope that helps a bit.