If you decide to hire a car, head north and enjoy the Jameos del Agua, then the Cuevas de los verdes, and stop at Arrieta beach bar for a bite (they're chicken in peppers, "Pollo con pimientos" is the best on the island) and a drink before visiting the Jardins de Cactus in Guatiza on your way back to the ship.

http://www.grupoestupendo.com

You can hire a taxi for the day €250 with no entry fee´s paid for.
The bus service is no use to visit Timanfaya etc.
You can use the cruise transfer to take you to within 10 mins of Arrecife, beware, its closed from 1330 until 1630.
Take the cruise line excursion, normally around €44 for half Island or use other tour companies.
Car hire firms are available but if you don´t know the Island you will miss lots of views.

The tour to include 2 entrances would be €60 per person.

Lanzarote is a beautiful Island, don´t miss the beauty.