Must-see, must-do for Greece? We have about 5 days, around 2nd week of July - port of entry, Athens. Want to do some island/s around. Please help! Thanks

I think there are thousands of different options for Greece depending on your interests. I'd start out by doing a search on the web, and looking through some guide books to see what you are interested in and then come back with some specific questions we can help you with.

If you want to do an island, I would pick one and split your time between that island and Athens.

Must do, in my opinion is Santorini

On your time frame, I would opt for one of the Saronic Islands.

You don't have much time. Santorini is fabulous but you'd need to fly into the island from Athens to save time. You could get the catamaran to Hydra, took us about an hour, maybe more. Hydra was lovely, there are no vehicles on the island (except a rubbish truck), lots of donkeys and you can walk easily around the centre and to swimming places. We stayed here and really liked it:
http://www.ippokampos.com/

That would give you a feel for Greece, Mykonos is another option. Mid July is peak season, you may have trouble getting flights and accommodation.

Kay