Foz do Douro beach
Praia dos Ingleses is a largely sandy beach with rocky outcrops around the shore line. Located in the upmarket Foz do Douro district of Porto this is cafe and promenading central. The cafes here stay open until late and Praia dos Ingleses is often still buzzing long after dark.
The beach itself isn't the greatest, but then you have to head out of Porto to find really nice beaches. I'm also a little skeptical of the Blue Flag status of this and some of the other beaches along this stretch of coast. The River Douro empties into the sea right next to this beach and that always used to be source of pollution. Add to that the oil refinery and terminal a few Km up the coast at Leça da Palmeira and I'm not entirely convinced...
Regardless of the water quality this is still a good spot to just hang out and observe Porto society and top up the tan whilst you're at it!