The Santorini Caldera cruise is offered by almost every travel agent on the island. There's various versions, some are half day, some are full days and some are sunset cruises.
It generally starts at the port and consists of a visit to the volcano (Nea Kameni), along with a trip to the hot springs on Palaia Kammeni, a stop off for something to eat or traditional Greek food on board the boat. Some further sailing round the islets that make up Santorini
where you can enjoy the volcanic rocks and scenery then it usually concludes with a chance to enjoy the sunset in Oia
The Caldera cruise is good for families, as long as the kids aren't too young and for nature lovers, it's probably best avoided if you get sea sick and don't forget your waterproof sun screen as you'll be on and off the boat all day.