Well then, I second A Coruna! Although it looks like there are some showers in the forecast, there are some wonderful beaches in the area. The Orzan and Riazor (site of the August music festival) beaches are most popular, but you can find a few others where you won't see much in the way of crowds and that have better views (in my opinion). There are a number of museums that, honestly, I have yet to visit (but the archaeology museum is at the top of my list!). Consider catching a soccer game at the stadium, even if you don't understand the game.
The Tower of Hercules is worth a visit -- oldest existing Roman lighthouse. A Coruna gets it's name from the crown of Geryon, who Hercules slew and then buried his head and crown. The myth goes on to say that he built the lighthouse over the burial to commemorate his 3-day battle with Geryon. The coat of arms for the town has an image of the lighthouse atop a skull and bones to represent this myth.
Maria Pita is local heroine who defended against the English Armada. Her story is worth looking up, although her string of husbands had a habit of prematurely, um, dying. Check out her statue in the city plaza. A festival named for Maria Pita goes all through August, so you might be able to catch some local festivities. Walk the promenade if you need some (more) exercise. It's a bit over 5 miles long and you'll pass the aquarium and some other cool attractions.
You may want to hop over to nearby Ferrol to check out the castle of San Felipe, some of the historic churches, and attractions related to Ferrol's nautical history, such as the Exponav museum (has a much longer official name). There are a few festivals and sporting events (surfing and equestrian) that happen in August in Ferrol.
