Planning first trip to Greece

My husband and I are hoping to travel to Greece with another couple. We are looking at dates in mid-May vs mid-July for about 10 days. Are there any pros or cons that stand out for these dates? Any must-see locations? We are hoping to island hop, swim and enjoy the local atmosphere. Is it too ambitious to pick 2-3 islands for this time period? Any advice for first time travelers would be appreciated!