Have you thought of hiring a taxi that may not be any more expensive than renting a car. Its worth checking out.
And I too would go with the taxi after you've researched using Aldrich's advice. The taxi man knows all the cruise tours so let him do the work & save yourself the hassle of driving.
HI, We hired a car on Lanzarote at a cost of 40 euros and then drove up to Fire Mountain which cost us 8 euros each for entry and a further 5 euros of (very cheap!) petrol, so all in all not really any cheaper than doing an excursion BUT we did have a great relaxed drive around a very unusual landscape, nice quiet roads and made a change from having to follow an itnerary. Good luck, Carol Guy.