In your place I would get off at the gua gua (wa wa) bus station. Lots of yellow local buses on your right and a beautiful theatre on your left just after a sharp turn off the main road. You will have seen the Cathedral on your left just before.
Then explore the Vegueta. It closes for siesta so get that done by Lunchtime.. There are some nice old colonial style buildings, now museums, but last time I went if you didn't at least read Spanish you are very limited.
Siesta time get a taxi - or a gua gua if you are brave - to Parque Santa Catalina. From there head for the Playa Canteras for lunch . Don't have a dessert as there are some super cake places along the beach. Shopping is nearby either at El Corte Ingles or the Muelle shopping centre by the cruise terminal. Then you can catch the bus all the way back from the bus station at the Muelle. Nice day out, you will not be able to do everything so either another day or better still another stay on the island.