If we can visit Greece only in the middle of summer I'd suggest visiting less crowded parts of the Peleponez, the northern part of the continent or the leading edge of islands with attention focused on the more distant spots. Out of season, especially between late October and late April you need to take into account a reduced number of ferry services to islands and quite skinny range of facilities (ie, equipment rooms, equipment). However, we will be able to find reasonable support for all major routes and at least one hotel and taverna open in the port and main city in all places, the smallest of the islands.