if picking up from lavrion marina, marina office can arrange a car for hire,we paid euro 20 a day whilst there,town is 5km away.
if picking up boat from lavrion port,town center is a short walk.
there is a small but reasonably stocked supermarket in the town center should be open till 9pm sat,and maybe sunday morning.
a few streets back from the town center there is also a fish
market and fresh veg stalls ,butcher and baker,may not be open on sunday
the marina only had a chandelery,and laundrette,and small workers cafe,open during day time.
the port area has a few bars and restaurants
most islands generally have reasonably priced/ stocked "spar"supermarkets,though maybe not what you used to in texas!