I stayed in Kefalonia when i was pregnant - about 5 months, i really didnt like it, there was 2 restaurants and no bars. I admit i wasnt looking for a boozy break/late nights but something would have been nicer.
My advice check how far you are from the beach ie outs was 45 mins walking up down an enormous bendy road.
Santorini would suit I think. Things to do and see - volcanoe, Akrotiri (a village buried by volcanic explosion like pompeii),various different beaches black volcanic sand, red sand and normal sand! Some great busy but not 18-30 type villages. As its fairly south of mainland greece you will get good weather in June ie hot and sunny but not too hot.
Hi, Lassi is definitely your best bet if you want some nightlife, although it is a big resort and might be a bit busy for what you want. We stay in Skala when we go and we love it. Great if you don't want hectic, but lots of bars and restaurants. There's a nightclub just out of town and you can easily catch a cab from the main square. Let me know if you want more details.