Since you don't want to take an excursion offered by the ship, you are going to be very happy with Isla de Margarita. There's a beach by the cruise pier!!! It's free and there's no need for taxis. Nice beach BUT would wear water shoes.
We just love Isla de Margarita! The locals seem to be happy to have us on their island and really try to please. We walked over to the beach above after returning from our ship's excursion the last time we were there. It seemed very nice! Nothing wrong with just going there for a day at the beach!!!!!!!!!!!!!!!!!!!!
Our last two times in Margarita we have taken an excursion offered by the cruise line called -- Coche Island Cruise. Both times it was wonderful (didn't need water shoes there)!!! You can tell we liked it since we did it twice!
Have Fun! WSEA