Hi Libby,
used to work in a local travel agents (sadly no longer trading!) but i would recommend only staying in athens for a couple of days and then going to the islands, kos is always a popular choice. Greek regional airline, Olympic ithink they are called did lots of flights to the islands very reasonable prices too.
I booked this type of holiday for my cousin and his girlfriend and they loved it, athens more of a siteseeing hol, and the the beach/family hol is the islands
hope this helps, would love to go to greece, deffo on my list of places to see!