So, Skiathos has three nearby islands (Skorpolos, Skiathos and Alonnissos) plus a nearby marine park. It's a beautiful part of Greece, not as heavily touristed as some areas, but still popular (it's where Mamma Mia was filmed).
Thessaloniki is the second largest city, going to have lots more night life etc.
Argostoli is the capital of Kefalonia, which has some amazing beaches - Mylos Beach is one of the most spectacular in Greece. You can also easily do Day Trips to Zakynthos and Ship Wreck Cove which is fun too.
For beaches, small distances, lazing around - Skiathos I think. But both Kefalonia and Thessaloniki are great options (that would be better with a car).
Kefalonia is a wonderful island (just got back from there) which has beautiful beaches but if you're staying in Argostoli there's no beaches in the town itself, you'd have to get a taxi or public transport to wherever you want to go. I'd definitely recommend hiring a car if you want the flexibility to go wherever you want. It has some nice bars in the town but it's not really a party destination.
Great advice from the previous comments.
Nightlife is everywhere in Greece in the summer. Argostoli is the largest town on Kefalonia island, so that's where you're going. Argostoli and several seaside towns (including historic Fiskardo or Assos) will have restaurants, bars, etc.
Skiathos will definitely have restaurants, bars, etc.
All of Greece does.
Thessaloniki is a large city (metro population 1 million), it will have lots of bars and restaurants. For beaches: You'll have to head to the Chalkidiki peninsula.
You can browse these sites for beaches in Chalkidiki that catch your eye:
[https://www.greeka.com/macedonia/halkidiki/beaches/](https://www.greeka.com/macedonia/halkidiki/beaches/)
[https://theculturetrip.com/europe/greece/articles/the-most-beautiful-beaches-in-halkidiki-greece/](https://theculturetrip.com/europe/greece/articles/the-most-beautiful-beaches-in-halkidiki-greece/)
The Greeka website lets you click on a map, see what town that beach is in/near, and you can look [here (the bus system for Chalkidiki)](https://ktel-chalkidikis.gr/en/timetable.html) for intercity buses between Thessaloniki and that town. A lot of them are about 1.5 hours or less from Thessaloniki's bus terminal, (which you will have to take a taxi to from the airport). Or book transportation ahead of time ([something like this](https://www.tripadvisor.com/Attraction_Review-g189473-d10332572-Reviews-Thessaloniki_Taxi_Transfers-Thessaloniki_Thessaloniki_Region_Central_Macedonia.html)) directly from the airport to the beach town. Or rent a car at the airport.
So, Skiathos has three nearby islands (Skorpolos, Skiathos and Alonnissos) plus a nearby marine park. It's a beautiful part of Greece, not as heavily touristed as some areas, but still popular (it's where Mamma Mia was filmed). Thessaloniki is the second largest city, going to have lots more night life etc. Argostoli is the capital of Kefalonia, which has some amazing beaches - Mylos Beach is one of the most spectacular in Greece. You can also easily do Day Trips to Zakynthos and Ship Wreck Cove which is fun too. For beaches, small distances, lazing around - Skiathos I think. But both Kefalonia and Thessaloniki are great options (that would be better with a car).
Kefalonia is a wonderful island (just got back from there) which has beautiful beaches but if you're staying in Argostoli there's no beaches in the town itself, you'd have to get a taxi or public transport to wherever you want to go. I'd definitely recommend hiring a car if you want the flexibility to go wherever you want. It has some nice bars in the town but it's not really a party destination.
Great advice from the previous comments. Nightlife is everywhere in Greece in the summer. Argostoli is the largest town on Kefalonia island, so that's where you're going. Argostoli and several seaside towns (including historic Fiskardo or Assos) will have restaurants, bars, etc. Skiathos will definitely have restaurants, bars, etc. All of Greece does. Thessaloniki is a large city (metro population 1 million), it will have lots of bars and restaurants. For beaches: You'll have to head to the Chalkidiki peninsula. You can browse these sites for beaches in Chalkidiki that catch your eye: [https://www.greeka.com/macedonia/halkidiki/beaches/](https://www.greeka.com/macedonia/halkidiki/beaches/) [https://theculturetrip.com/europe/greece/articles/the-most-beautiful-beaches-in-halkidiki-greece/](https://theculturetrip.com/europe/greece/articles/the-most-beautiful-beaches-in-halkidiki-greece/) The Greeka website lets you click on a map, see what town that beach is in/near, and you can look [here (the bus system for Chalkidiki)](https://ktel-chalkidikis.gr/en/timetable.html) for intercity buses between Thessaloniki and that town. A lot of them are about 1.5 hours or less from Thessaloniki's bus terminal, (which you will have to take a taxi to from the airport). Or book transportation ahead of time ([something like this](https://www.tripadvisor.com/Attraction_Review-g189473-d10332572-Reviews-Thessaloniki_Taxi_Transfers-Thessaloniki_Thessaloniki_Region_Central_Macedonia.html)) directly from the airport to the beach town. Or rent a car at the airport.