Cádiz is reowned throughout Andalucía for it's long sandy beach and historic sites - most notably the Cathedral, Roman amphitheatre and the Cárcel Real (Royal Prison). All of these sites can easily be seen in 1-2 hours so you can spend the rest of the day at the beach, making Cádiz a worthwhile daytrip for families with children.
Cádiz is 125km (77 miles) south of Seville. Trains leave almost every hour from Santa Justa train station. Journey is 1 hour;
www.andalucia.com/travel/trains/home.htm
There are also regular buses from the main bus station in Plaza de Armas; www.andalucia.com/travel/bus/home.htm
We travelled to Cadiz on the train but it took us just under two hours and it cost 24 euros return - worth every penny/cent.