Greece suggested itinerary for July
 

We plan to travel to Greece in July'13 with a couple of young kids.

Would like to explore beaches and some scenic spots on the islands.

We would like to visit the following places.

1. Athens 2 days

2. Meteora 1 day

3. Santorini 2 days

4. Paros/Naxos/Mykonos 2 days.( which island is preferred).

We have kept 1 day for traveling from athens- meteora and back and another day for traveling from athens to santorini.

We will fly out from one of the islands via athens to istanbul .

Hence we have a total of 9 days.

Read some trip reports and have a brief info on some of the islands.

Please suggest an itinerary which will not be too hectic.

On 3 nights/2 FULL days you may see most of outdoors ancient sites and maybe visit one or two museums, not much more. 2 nights/1 full day or 1 night/departing late afternoon will allow you to see some of the Meteora monasteries. 3 nights/2 full days is rushed even on the tiniest island that Santorini is but not sure what your aim is on each location... If by 9 days you mean 8 9 nights/8 full days,I would stick with Athens+Meteora+1 island at the very most and still this would be rushed.
Are you arriving on Athens airport? Is it an early morning arrival or what? If Santorini is a must for you (it is for most first time visitors, not my preference, but so be it...) I would see if I could get a connecting flight to Santorini. Spend 2 or 3 nights there, then ferry or flight to Athens and train to Meteora. 1 or 2 nights on Meteora, return to Athens for rest of the days before flying to Istanbul. If a second island is a must, drop Meteora. On July I vote for Naxos or Paros rather than Mykonos, but this is up to you.

"On 3 nights/2 FULL days you may see most of outdoors ancient sites and maybe visit one or two museums, not much more."
In Athens that is ....

The general wisdom is to visit island(s) first and see Athens last so you're there already for your departure flight.

I agree with the advice above... pick one island and make that island anything BUT Mykonos. It will be packed in July.

You also need to consider your trip to Meteora... you say you have budgeted a day for transportation... it is pretty much a day each way so you need to budget for 2 days. I would spend more time on the mainland but that's because I am more interested in antiquities than beaches.

IMO way too hectic - I know nothing about Meteroa but I'd do Athens and one island max. My personal choice would be either Santorini or Naxos depending on how laid back you want to be. It will be HOT! which also would motiviate me to get somewhere and stay there.

Mykonos and Santorini will be packed with tourists. I'd pick another island. Meteora from Athens is a two day trip. Delphi or Mycanae/Epidaurus are each one day trips.
As someone commented above, fly out to one island and come back and visit Athens and possibly do a one day trip. I have visited Athens eight times and still have not seen everything. There are a number of interesting museums, good cafes, & ancient sites. I also go in October and November when it cooler. If heat bothers you, you may have to take a slower pace. And Rhodes, Crete or Corfu are my preferred island. And, they are hot and crowded too.

The heat doesn't stop millions of tourists from visiting Greece in July and August..
Plus during these months there are northern winds ( the so called meltemi winds) especially on the aegean islands.....

If i am not wrong, half of the USA has a climate that is not only hotter but also humid... ( Greek climate in the summer is extremely dry)....

You're right, clausar — I used to live in one of those places — Houston, Texas, which has both heat and humidity. How people managed to survive there in the days before air conditioning I can only imagine, but they did. I seem to remember people spending little time outdoors in the summer, preferring to stay in their air conditioned environments. A car sitting in the sun was like an oven until the A/C kicked in.

Clausar is correct that the dry heat of Greece is much more bearable. Greeks adapt to the summer heat by starting their day early in the morning, then taking a few hours break in mid-afternoon. Things liven up again in the evening, with shops staying open late, and restaurants not getting busy until around 9pm. Tourists might take a lesson from the Greeks. ;-)

Another way many Greeks cope with high summer temperatures is to take their holidays on the islands, where temperatures can be 10 C (18 F) degrees cooler than in Athens. :-)