There are two tiny islands next to Santorini, both are uninhabited, except for a church on one island, and there are no ferries to these islands. Have a look at the satellite image of the islands on Google Maps for a better view.
All the Greek Islands are friendly, most are touristy and, as far as the locals will be concerned, you will always be considered a tourist. You don't mention what time of year you will be travelling there, but you need to be aware that there are fewer ferries in the colder months and
ferries are often cancelled then due to storms.
Camping in beaches is not a good idea, and you are likely to be moved on by authorities. If you are not an EU citizen, a condition if entry to Europe is that you have enough money for your entire trip. Do some research on camping grounds, there is plenty of information on Google or you could get a tourist guidebook.
Also, you don't say how long this trip is going to take. But, if you are a non-EU citizen, you are limited to 90 days out of every 180 days in the Schengen Zone.
Each and every single Greek island has many aspects worth visiting it. The personal interests you may have may be the prime reason for visitng anyone of them.
For island hopping the Cycladic islands aree the best since they have daily connections between them and is easy to travel from the one to the other. Many people prefer the less tourist frequented ones while others prefer the mainstream ones. A combination can be Syros, Tinos, Naxos, Mykonos, Santorini and then Patmos and Rhodes. and/or Kos. Of course if you have the Money and the time a lot more can be visited and apart of the fares the cost in most of them is far less than the trendy ones.
Camping on the beach is not officially allowed but if you spend one night in a rather secluded one, you will not face much of a problem, provided thet you are aware of all dangers involved. Such as a a dog coming and licking your face during the night or some drunken person want to make you company.
Best bet is to buy the current year's book " Greek Island Hopping " author Frewin Poffley , there is a small website which tells you a little more , but basically it is a first class guide to ALL the islands bigger than rocks , and ALL the ferries , AND is a very good guide to Mainland Greece and all things Greek .
Nearly all of Greece is friendly , to the point where if you do meet an unfriendly Greek , that can come as quite a shock .
The smaller and remoter islands see less mass tourism .
Campsites are few , facilities tend to be a bit dusty and shabby , and the pitches are often uncomfortable gravel
The guidebook above will tell you all you need to know .
Camping on the beaches is illegal , and unless you can get somewhere well out of sight , away from dwellings , you are quite likely to be chased off by the Police , who in some instances confiscate your passport .
Tip is not to actually set up camp until dusk , go to bed with the dark , DONT flash lights about or make any noise ( Greek dogs can hear a suppressed cough from a mile away ) , rise with the dawn , clear all signs of your presence away , and move on .
There are two more fragments of the ancient volcano that is Santorini as well as the main isle ; - all are heavily touristed and devoid of discreet free campsites , so best bite the bullet and pay 50 to 100 euro per night for a room . Accommodation touts with minibuses meet the ferry boats .
The small island to the east is Anafi , with infrequent ferry links ( the book will tell you the situation for 2015 ) a small population , and a very remote small-island feel .
If you want the small-island Zorba-the-Greek experience then get a ferry from Pireaus down to Naxos ( the book has the times ) that lands you at the island capital , accommodation touts meet the disembarking passengers at the town end of the quay , nice old medieval maze of an old town to explore , miles of safe sand beaches , and real mountains you can ride up into on the local island buses ( station at the town end of the ferry quay )
From the next jetty along the waterfront there , you can take the small " Skopeliti " ferry , 6 days out of 7 at 2.00pm , and take yourself on a cruise to the " Little Cyclades " isles ( Iraklia , Schinoussa , Koufounisi , lonely Donoussa , and then on to the two harbours in Amorgos )
Schinoussa and Donoussa are probably the least touristy , and suitable for overnight bivouacking for free .
Tickets 12 euro Naxos to Schinoussa , from Zas travel shops along the Naxos waterfront strip .
I think the best Hostel in Greece is in Plakias on the south shore of Crete . ( google Youth Hostel Plakias , and look at You-Tube and pics / vids on Google-earth )
To get there ferry or plane to Xania or Heraklion , airport bus to town bus station , then Hourly express coach along the north coast highway to Rethymnon old town bus station ( stay in the nice Hostel there , closeby - see Rethymnon Youth Hostel website ) and then 6 local buses a day go across the island through spectacular mountain scenery 20 miles south to Plakias .
Both hostels charge only 60 euro a week , and the one at Plakias is Idyllic and a very good crowd there , no chavs .
If you are planning for Island hopping in Greece, its very much possible. I am not sure whether you can camp on the beaches or not. As you can't afford hostels as this will be at the end of your trip. In that case you can stay in hotels which are affordable.
Forget beach camping. Greece is full of semi-stray dogs who seem drawn to any westerner doing anything unusual. Lie down on a beach or in a field and soon one or more dogs will come and try to play with you.
Many of these dogs do have homes - they just seem to like latching on to visitors. One persistent mongrel followed us about for 4 days, sleeping on the doorstep of our appartment.
Truly one of the best ways to see Greece is to plan a Greek Island Hopping Vacation. This can be done by air, by sea or a combination of the two. Budget and timing are a serious consideration as they will dictate how many different islands you can see, and what class of hotels you should be looking at. Know that the islands you choose will also make a difference in pricing as some are more visited, thus more expensive, while others are lesser known and less expensive.
Here are some things to consider before getting started; destinations you are interested in, the different types of transportation available, choosing your hotels and then finally planning some sightseeing and fun activities.
Start with your budget and dates. A reasonable budget, for a couple traveling from the USA or Canada to Greece and staying for one week, would be about
