Nothing to do - stay on the ship.
Poor Mr Marsh - what an unadventurous life....Olbia has a good beach easy which is to get to - only a very short walk from the ships tender point. We tendered in so would imagine you will too. We found the beach to be clean and very safe. Near Olbia there are several fantastic beaches, rent a car and go-my favourite is Budoni's beaches, only 30m south of Olbia... If you don't want to drive go into the port terminal building. There is a desk there that will help you with taxi service. We paid 25 Euros each way to go to nearby Pittulongu beach, which was beautiful. We rented chairs and umbrellas (about 6 euros each) and had a lovely and relaxing 4 hours there, before being picked up again by the taxi service ( we arranged in advance what time they would be back). It couldn't have been easier or more relaxing. Downtown Olbia does not have much to offer at all.