Nidri on Lefkas is good, Samos is lovely, completely different to Kefalonia though.
Mykonos has some fabulous beaches and the town is great for restaurants, bars etc. Never been to Skiathos so can't comment.
Crete is also lovely, especially in the Chania area.
Where did you stay in Kefalonia?

We stayed in Lassi and Sami. Enjoyed both but liked Lassi because the beach was so close.

How is Samos different?

I really liked the sceneray and beaches in Kefalonia and general pace of life so looking for something similar. Did find that a lot of the restuarants were catering for the English tourist, so would be nice to go somewhere a little bit more authentic too.

I would suggest Skiathos. It hasn't got the rugged beauty of Kefalonia but loads of gorgeous beaches with resorts on hand. the liveliest place on the island is Skiathos town and the small strip with the nightclubs is in the harbour area away from the accommodation.
All of the coastal resorts have plenty on offer and the bus service into Skiathos town is excellent, although the closer to the capital you stay the more chance you have of being left as they are normally packed! Taxis are reasonable though and there are plenty around.
Nowhere on Skiathos will you find the 18- 30 types. It is a laid back cosmopolitan island and great for relaxing. (Troulos and Koukanaries are my personal favourite resorts)
I love Kefalonia myself and i am going back for my 9th visit in June. I also love Skiathos and am going back for my 4th visit in September. I'm sure you will fall in love with it too!

John

Like John, I would recommend Skiathos.
I love the Ionnian islands, and have visited them many times. I find the Sporades are very similar and if its the ruggedness that you like you will find it on the Sporades islands espescially if you travel over to Skopolos and Allonisos, but they are much quieter than Skiathos.

I like staying in Skiathos town and travelling out to the beaches and villages on the bus route in the evening - avoiding the crush on the bus by starting in town. If you like the nightlife its here if you want it, clubs out by the airport, music bars and nice cafes tavernas in town around the harbour and back streets.

Whatever you want you will find it here, sometimes you just need to explore a little. Get away from the main road and over to the north of the island. You will be surprised by the island and what you find.

You will know if the magic takes you, Skiathos is known as the boomerang island, once you leave you always want to come back .

I did not find Skiathos very good unless you are a beach person which their beaches are beautiful, Unfortunatly once you have visited Kefalonia it's hard to find somewhere else but have heard very good reports on Parga cannot speak personnally, I have visited many Greek Islands over the last 20 years, some I've enjoyed some not so like Alonysos but as it has been so long since I visited them things could have changed to Lesvos and Syros stand out in my mind
pojo