For fun and frolics in the sea, your best bet is the Costa Del Sol or the Costa Blanca. Prices in these areas of Spain are so good that you won't quite believe it. You will also find that the temperatures in these areas are incredibly high throughout the summer months, and as such, if you are planning a very late season holiday, you can still expect beach weather.
The season is long throughout Spain really, so no matter where you are looking, you are sure to find warm weather throughout September, and in many cases through October and November too. To find these cheap holidays you do not have to look too far and you do not have to look particularly hard either. The one thing to really try to remember though is that you will find a better priced holiday if you steer clear of the package holidays and go for individual bookings. Try to find
Hotels in Spain without booking the flight with them, and then try to look for separate flights afterwards. Doing this will result in your ending up with a cheaper holiday overall, as long as you have taken care to find the best deals for both your hotel and your flights