Sifnos I think would be excellent for a honeymoon, if you wanted a quiet time, and didn't want too luxurious accommodations. Beautiful island for walking, wonderful old town of Kastro, good beaches (though not as good as some other islands.) Some of best reastaurants in Greece.

Spetses is a bit livelier, but the tourists are largely Greek, so it's kept its old atmosphere more than some places. It has a wide range of accommodations, including some top class ones, which have web sites. There's a famous old hotel, the Poseidonion, which supposedly is being refurbished to its Edwardian splendor, though I'm not sure when it will be open. Also some excellent restaurants.

Hydra remains wonderfully atmospheric despite the tourists, and has good restaurants. No beaches to speak of, though there is pleasant swimming off the rocks, and not much to "do," but it's a nice place to just sit around, and you can easily make day trips to the neighboring islands. Good restaurants and cafes.

All the above are described in most guidebooks and are easy to get to from Athens.

Others may differ, but I wouldn't go to Corfu: lager lout city, though it might be a good place to visit if you're in training for soccer hooliganism. Rhodes is nice as long as you stay in Rhodes Old Town and avoid Faliraki and other package resorts which resemble something that might have been created by Pasolini in a particularly bad mood.