Bilateral agreements are based on the "freedoms of the air", a group of generalized traffic rights ranging from the freedom to overfly a country to the freedom to provide domestic flights within a country (a very rarely granted right known as cabotage). Most agreements permit airlines to fly from their home country to designated airports in the other country: some also extend the freedom to provide continuing service to a third country, or to another destination in the other country while carrying passengers from overseas.
Zakynthos, or Zante, has shrugged off its reputation as a destination for lads on tour (as long as you avoid Lagana and the built-up south coast) by rebranding itself as Greece's greenest island. It's not just the emerald hills sliding into the electric blue Ionian: much of the south coast is a nature reserve where endangered loggerhead turtles hatch in the sand. The turtle beaches are off limits, but there are countless coves in every hue of green and blue. Favourites are tiny Xigia, with its bubbling underwater springs, and craggy Porto Limnionas, with sunbeds wedged between the rocks and palm-frond umbrellas positioned between the pine trees. Skinari is the starting point for boat trips to the most famous landmarks, the Blue Caves and Shipwreck Beach, where a rusting liner leans into the chalky cliffs. From Keri, you can cast away for Marathonisi island, another turtle sanctuary.
As a starter to explore ferry services, go to the Ferryhopper.com and punch in your route searches. Note: air travel between islands is not generally good. Air travel is normally only feasible between Athens (or other mainland destinations) and the island in question. Some exceptions do exist – Rhodes Kastellorizo, Kos-Leros-Astypalaia, Thessaloniki-Limnos-Ikaria etc. but again you will need bookings weeks in advance.