There are a wide range of activities on the islands from horseback riding to ATV rentals to scuba diving.
Activities do not need to be arranged before you arrive in Greece.  However, we recommend speaking to the captain when you arrive.  If any activities do need to be booked in advance the captain will phone ahead while on charter.
Recent law changes have made it more difficult for a US citizen to rent vehicles in Greece.  Many places are now requiring you have an IDP (International Drivers Permit).  While some smaller rental places may not require this license, we highly recommend getting one prior to your journey if you’re planning to rent a vehicle, ATV or scooter in Greece.  Luckily it is easy and inexpensive to attain an International Drivers Permit.  The best method we’ve found is through AAA.