I recommend you not to stay in Sevilla at weekends in summer. Everybody goes outside, to the beach or “a la sierra”, and it becomes a ghost city on Sundays. Huelva city is 2/3 hours by bus from Cádiz city and it has no beach, so it’s better you visit Chipiona, just 60 kms from Cádiz and well comunicated to Jerez (hourly buses), Cádiz and Sevilla.
Although there are many hostales and hotels in Chipiona, you will need to book before. This town has lots of people in August. Check
www.chipiona.net http://www.chipiona.org/ciudad/alojamientos.htm As Eddie says, Sanlúcar is a good option but the beach is not so good. And don’t forget to be close to the water!! It is usually hot.