Alright, so I plan on traveling somewhere between July 1st through July 17th.
I plan to live out of a carry on for the entire duration and my budget is ~$2000, USD (not including airline fees) for those 2 weeks.
Ill also be traveling alone but I plan on staying at hostels or cheap places for travelers (AirB&B maybe?).
Im looking for a little bit of everything in this trip, and by that I mean I would like some days to background the people and culture then enjoy the night life. While other times go on a hike and enjoy breathtaking views and watch the sunset. It would also be nice if there are moments during the night where I can go outside to a dark area and stargraze.
I dont like going to museums when the place is just a modern building filled with collections, however I do enjoy visiting ancient monuments and architect or temples (if that makes any sense?).

With that said, its why Ive lately been really eyeing Greece a lot (specifically the islands, although Im open to ideas based on what Im looking for above). So I just had a few questions before I go through with buying the airplane soon

Is, 2weeks too long or just enough time to enjoy the Greek Islands?
I was even thinking maybe 1 week in the greek islands and another week go to South Italy or Istanbul? pretty much another location somewhere nearby.
Ideally, it would be nice if I can book my hostels/ferry tickets the day of (or before) so Im free to make my plan as I go. Would this be feasible or will I expect to have everything fully booked and triple price if I do this? What would you recommend?
I would also like to just buy a one way ticket to Athens (arrival), and wait to buy my return ticket home (home being California) sometime during my trip in case I do decide to visit another location. Generally speaking how much more do I expect to pay if I buy my ticket last minute?
Im considering to bring a cheap phone for quick access to information. If I bring an unlocked GSM phone, is it pretty easy/reliable to buy a SIM card and use data that way? If so, is the data there pretty cheap for browsing the web in case I need to look up nearby hostels?

Thats pretty much the few items holding me back from actually buying a ticket. Based on everything mentioned so far, any ideas? Am I underbudget or anything concerning?