You can take a train to Denia from the Benidorm train station for about 5 Euro return.  The trains leave about once per hour in both directions.  Denia is a nice town with an amazing old castle to climb around (entry is about 3 Euro for adults, 1 Euro for kids).  A relaxed day can be spent walking around downtown, eating lunch at a local restaurant, and looking at the boats in the harbor.  Denia is a nice break from the tourist-y feel of Benidorm.  It is not exciting but makes you feel like you are in Spain, not England.