Itinerary signoff for Greece

It's always a risk to schedule a return to Athens from one of the islands on the same day that you plan to fly out of the country. Cancellations are rare but delays do happen. I strongly urge you to return to Athens at least one day before your flight to Budapest. Better still, re-arrange your itinerary. When you first arrive in Greece go directly to your first island and save ALL of your time on the mainland for the last part of your Greek adventure. That way you aren't backtracking and you eliminate any chance of a ferry or plane problem causing you to miss your onward flight.

I've been to most of the places on your itinerary, and was in Greece and Turkey earlier this year. We used to travel as far and fast possible in our 3 week trips, but $9 per U.S. gallon Italian gasoline brought us around to traveling a better way--slower.
You may have overestimated the distances between cities in Europe, as you're going to be spending so much time traveling from city to city--with so many stops to make. We always prefer to travel in straight lines to cities that are relatively close to each other and to cities that compliment each other. One great trip was Budapest-Bratislava-Vienna-Prague. That could be have been lengthened to Dresden and Berlin if time allowed.
For one, I wouldn't care to spend 12 days in Greece, despite the islands being beautiful. Other places that don't really interest me are Luxembourg and Belgium.
And as the Summer goes on, I don't have the desire to travel in the south--as it's too hot and air conditioning is not always great. We took a Baltic cruise which was an absolute bargain and included 2 days in St. Petersburg.
You really should reconsider a total itinerary that flows a little better--not jumping around and going in circles. Or, just roll with the flow after you get there--letting your trip take a life of its own without planning weeks ahead.

Thanks @Lee .... i can definitely consider going to islands first and then explore mainland. However, i was giving some 4-5 hrs buffer in between my two flights to give some space for flight delays.

Thanks @David I tried to avoid any backtracking or going in circles for inter cities. As per my current plan
Athen -> Budapest -> Cracow -> Prague -> Hallstatt -> Venice -> Rome -> Barcelona (and other places in Spain) -> Lisbon -> Paris -> Amsterdam -> Belgium (only if will get Tomorrowland tickets). Any feedback is really appreciated.

Frankly speaking, I would love to go little unplanned but flight tickets and other hotel or hostels will be really expensive at the last moment. I am really interested to know how people travel in an unplanned way in Europe.

Google Maps url for reference :
https://www.google.ie/maps/dir/Budapest,+Hungary/Kraków,+Poland/Prague,+Czech+Republic/Hallstatt,+Austria/Venice,+Italy/Rome,+Italy/Barcelona,+Spain/Lissabon,+Portugal/Paris,+France/Amsterdam,+Netherlands/@44.7950332,4.9468155,6z/data=!4m62!4m61!1m5!1m1!1s0x4741c334d1d4cfc9:0x400c4290c1e1160!2m2!1d19.040235!2d47.497912!1m5!1m1!1s0x471644c0354e18d1:0xb46bb6b576478abf!2m2!1d19.9449799!2d50.0646501!1m5!1m1!1s0x470b939c0970798b:0x400af0f66164090!2m2!1d14.4378005!2d50.0755381!1m5!1m1!1s0x4771366f6e414663:0x16c165596a26c1ad!2m2!1d13.6492617!2d47.5622342!1m5!1m1!1s0x477eb1daf1d63d89:0x7ba3c6f0bd92102f!2m2!1d12.3155151!2d45.4408474!1m5!1m1!1s0x132f6196f9928ebb:0xb90f770693656e38!2m2!1d12.4963655!2d41.9027835!1m5!1m1!1s0x12a49816718e30e5:0x44b0fb3d4f47660a!2m2!1d2.1734035!2d41.3850639!1m5!1m1!1s0xd19331a61e4f33b:0x400ebbde49036d0!2m2!1d-9.1393366!2d38.7222524!1m5!1m1!1s0x47e66e1f06e2b70f:0x40b82c3688c9460!2m2!1d2.3522219!2d48.856614!1m5!1m1!1s0x47c63fb5949a7755:0x6600fd4cb7c0af8d!2m2!1d4.8951679!2d52.3702157!3e0?hl=en

You have chosen what I believe are Greece's two most tourist-overrun islands, which would not be my choice, but I can't deny that they are picturesque. Be prepared to have some difficulty getting photographs not full of other tourists, though.

I can tell you how I travel relatively economically as a spur-of-the-moment traveller: I cover a much more limited area than you are attempting, so even last-minute rail tickets aren't terribly expensive. I spent 89 days in Spain this summer, and I did not get to Andalucia. If you're not currently in a position to take a long trip every year, I can understand the desire to country-hop, which does tend to run up your transportation costs if you don't pre-purchase your flights and train tickets. Do keep in mind that bus tickets usually don't increase much in price if bought only a day or so ahead, and on occasion the bus can be almost as fast as the train.

Another productive technique is to research possible day-trip destinaions near each base city and add one or two extra nights at most of your hotels. Once in the city you can decide whether you want to spend all the time there or would like a change of scenery for a day. Your trip is very heavily weighted toward major/capital cities. I think you'll have a much more memorable (and not so exhausting) trip if you include more small cities and towns. And having some potential alternative plans can be very handy if you run into a period of uncooperative weather.

I strongly encourage you to cut Spain and Portugal out of this trip plan and visit them later. That would give you a good chunk of time to distribute among the remaining countries. Only seven days in France is pitifully short, and 15 days in Italy will not allow time for much more than the standard destinations of Rome-Florence-Venice. As a photographer you'll probably enjoy seeing some places that don't show up in every othrt traveler's photographs.

I think yoh can dispense with Luxembourg on this trip and use the time to expand the range of what you experience elsewhere.

Croatia is lovely, but I wouldn't try to cram it into this trip. A second trip could combine Croatia with Slovenia, Romania, Bulgaria, Serbia, etc.

And what happened to Germany? You definitely will need to return to Europe. Germany and Luxembourg (plus possibly Switzerland or part of Scandinavia) could work together.

Abhit.lita, regarding your plan to fly to mainland, and then out, in same day. Don't. Yes you say you allow 4-5 hours. What happens if your plane has a mechanical malfunction? There is not an unlimited fleet of planes, so flight will be cancelled. Do what you are advised to do .... put the island(s) FIRST. Other good reasons to do so: (1) you're coming a looong way to reach Athens, so you'll be TIRED on arrival. Rather than rush into intense big-city sightseeing, better to go to an island ... getting over travel exhaustion is much easier, more relaxing on a lovely isle. (2) by doing Athens last, you give yourself a few days to get used to Greek customs, signage, ways of doing things, & you're more prepared to deal with a vibrant, busy major capital.

I also agree that you've chosen the two MOST touristic/crowded/expensive islands. Santorini, because of its famous Caldera View, is usually considered a "must-see." However, Mykonos' fame comes more from its "party" reputation. It is a dry island with not much landscape for a travel photographer. Why not choose Naxos? It has amazing variety of landscapes... from beautiful beaches to dramatic interior mountains, and ancient ruins. Your easiest most time-efficient sequence would be: fly to Santorini, ferry to Naxos or Paros, ferry to mainland -- travel to Meteora & back -- Final days in Athens.