The weather in October in Athens is cooler than June, July, August and September.
If you follow the below link you will find the answer to your question about nice hotels:
Firstly let me reassure you that it is quite ordinary for tourists to cruise islands in October.
Some ideas about daytrips to islands nearby Athens are:
Hydra, Spetses, Aegina, Poros, Agistri.
Your best choice to get into the superb Islands nearby Athens, i.e. Hydra, Spetses, Aegina, Agistri, etc should be Pireaus port.
The journey to those islands range from 45 minutes to about two hours by hydrofoil, but it takes longer if you take the ferry.
My favourite of those Islands are Spetses and Hydra but they are also the ones which are further away from athens, i.e. Spetses by a hydrofoil (about 2-3 hrs), (the travelling time varies depending on how many stops the hydrofoil makes on the way).
Hydra by a hydrofoil, you will need around an hour and a half journey from Piraeus again depending on how many stops the hydrofoil makes on the way, but the Ferry takes 3 hours and 10 minutes with stops at Aegina and Poros.
Aegina is the nearest Island to Pireus and has a nice archeological area in Aphaia, with a Late Archaic temple in good preservation:
Hydra and spetses are quite cosmopolitan, so do not be at all surpised if you see any well known faces walking around in the streets, as many celebrities from Greece and abroad own houses there.
Hydra and Spetses towns are lovely places for strolling around as many of their buildings are built around the harbour by the sea and you will find some amazing palces to eat Greek and international food and some lovely places to enjoy a drink by the sea.
Alternatively you can enjoy daytrips or trips to world famous cites such as:
and of course
I hope this helps