Even though it is mostly rocky, Lanzarote also has beaches (if it didn't it would not be the popular holiday place that it is!).
They are quite varied. From the wild, black sand beach of El Golfo, which you have already seen, to the urban beaches in Puerto del Carmen (the beach in Los Pocillos is very big and pleasant), Costa Teguise, etc. to other gems.
I did not have the chance to see what arguably is the most beautiful beach of the island, Playa Papagayo (Google it!), but I have seen a couple of other interesting places.
The first is the wind-battered beach in Famara, enormously long and deep, frequented by surfers and dominated by the Risco de Famara,, a 500m high cliff that estends for kilometers to the nothern tip of the island. I can only offer a mediocre image of the place, I was mostly playing with my daughter at the time.
Another very interesting place are the beaches near Orzola, in the far north.
Here black lava flows from the volcano La Corona meet the sea forming sheltered coves. And within them, and slightly more inland, are low dunes of white sand. Very impressive.
This is one of my favourite places, and also a good spot for night photography, free from light pollution, since further North, looking toward the low-lying North Star, there is only the sea.