If you dont want nightlife/nightclubs then i would say Blanes is the place to go.
We visited Blanes a few years back and found it to be really relaxing , the hotel we used was right on the front we had rooms looking across the beach and out to sea.

As for the weather September can be a bit hit and miss like most places but i wouldn't let the weather stop me from going

Enjoy your holiday.

http://www.blanes.net/english may help

Not been there In Sept but there should be plenty open I would have thought.

Try the bahia Restaurant on the front between the rock and the port. The head waiter is a real hoot. A chap called Abundio.