The bus service on the island is too infrequent to be much use to tourists. Unless you have arrived on your own yacht, in which case you'll have no problem getting to most parts, you need a car or bike if you plan to get around. There are car ferries from the mainland, and many car hire places in towns, though prices vary. Although all travel operators are against motorcycle hire, as long as you have some bike experience, renting a 100cc scooter for the duration of your stay can work out very reasonable. Just make sure you check the bike out before you hire it. Most of the hire places are in Lassi, the main resort, and the capital Argostoli. Taxis are fairly reasonable and individual arrangements can be made with drivers to pick you up at specified times from beaches etc. They are usually helpful and friendly.
