You must go. Yes I know it feels a bit desperate to visit a cactus garden for something to do, yet I can tell you that you will be amazed and happy you did! http://lanzarote.com/jardin-de-cactus
The huge botanical garden (designed by Cesar Manrique, of course) houses the late botanist Estanislao González Ferrer's grand collection of cacti from America, Madagascar and the Canary Islands. They are landscaped amongst some of Cesar's sculptures in the dramatic setting of an old volcanic ash quarry. Volcanic ash is used by farmers to cover crops to retain moisture from the early morning dew.
We looked for the cochineal beetle that feeds on one variety of cactus. No luck. Perhaps it was the wrong time of year. The whole place is well kept and there is an interesting little gift shop. There is also a restaurant where the waiters can quickly come up with a first aid box, if you do as I did and lacerate your calf on a cactus when you step back for a photo.
Once again there are very clean public toilets. They also benefit from having more creative signage than normal.