I have visited Erikousa for a few hours on the trip from San Stef harbour. The boat trip which goes along the coast to Sidari 1st is stunning and well worth it but I believe last summer this starting point had to be abandoned due to the harbour silting up and only Sidari was used.
We only had 4 hours on Erikousa which is hardly long enough to expore very far and most people stayed on the lovely sandy beach.
I prefer to look around rather than beach lying but found although I walked mostly uphill through forest for quite a while I didn't reach anywhere with a view before it was time to go back. I think Phil is right in suggesting that an overnight stay in the hotel is needed to get the best out of the island. I am sure the same would apply to Mathraki and certainly for Othoni which is further out.
The Erikousa Hotel is quite small and guests of course rely on the sea conditions and weather being kind or it can be difficult to guarantee getting there and back again on schedule. So don't arrange your trip at the end of your holiday or you might miss the flight home!
Here are some photos of Erikousa trip from San Stefanos harbour:
http://www.agiosgeorgios.co.uk/erikousa_trip.htmMy friend Yiannis has a brother living on Othoni and he says it is a lovely island and well worth visiting.
Last summer, Fondas, the boy from the boats in Agios Georgios showed me his superb video of snorkeling around the rocky coast Othoni and its sea caves. The water was crystal clear.
PS If you want to go then do so before it is too late. If the windfarm proposal gets approved you won't see these island for the forest of tall wind turbines all around them.
Dennis