Cyclades vs Ionian vs Sporades ?

This guidebook, although 2 years old, will give you all the information you need to make an informed decision. Greek Island Hopping 2014 by Frewin Poffley (ISBN: 9781848486027)

I've not been to any of the Cyclades or Ionian islands so can't speak to those. I've been to some of the Saronic islands (Hydra, Aegina, Poros, Spetses) and some of the Sporades (both Skiathos and Skopelos). I'm not too much into nightlife so I also can't speak to that but for what you're looking for I would say Skiathos and Skopelos are good choices. I'm more into sightseeing and photography. I loved Skiathos especially, loved the town (dining and shopping) and had a lovely beach not far from our hotel. I don't remember seeing any kayakers. I have to say that of all the islands we stayed on or visited my favorite was Hydra for the scenery, the walking, the eating and drinking, the shopping (loved the art galleries and studios), the car-free atmosphere - loved it all. My favorite for relaxing, scenic photography, swimming beaches was Spetses. I would rank Skiathos right up there with those two but it was much busier, hectic - although not as much as Santorini or Mykonos from what I hear.

Everybody has their favorites so it's hard to recommend to someone you don't know. I would highly recommend (as the other posted stated) getting a good guidebook to the islands and doing a lot of research on the internet. Be sure to research the transportation options to the various islands and their frequency in September, it's shoulder season there so does quiet down a bit.

I have been to the a number of the Cyclades and Dodocanese Islands and loved them all. Some of the smaller and less touristed ones would be Sifnos, Milos, Amorgos, Astypalea, Ikaria which still retain a lot of their Greekness. Naxos offers an excellent combination of Old & New World Greece. Early September will be just after the High Season so there'll be less tourists, lower costs and still nice weather.

I've never been to the Ionian or Sporades Islands so can't comment on that.

Two books I highly recommend would be Greek Island Hopping (Frewin Poffley, 2010) and Blue Guide to the Aegean (Somerset, 2010) with Blue Guide being far better and more informative. Please note Blue Guide just covers the Aegean Islands but is filled with information, details, history/archeological sites, monestaries, churches, out of the way areas that Greek Island Hopping doesn't cover and fills the bill for those seeking more than lying on a beach, partying and shopping, which is not a bad thing. GIH is more for tourists but still is valuable.

There are so many Greek Islands to pick from. Check out Matt's Greek Guide for everything you need to know about Greece:

http://www.greektravel.com/

Thank you all for the comments and advice. Much appreciated! Will definitely look into getting a guide book to do some research