We book with either Maria at Kappa/Sidari or Noula at San Stef. Both are very helpful and will look after you. Their prices are much of a muchness when compared to each other. They are cheaper than Thomson IMHO.
Me, I enjoy the day trip to the isle of Eriskoua. SS harbour around 9.30 and then sail to Sidari for pick-up and arrive in Eriskoua around 11. Return sail around 15.30 - back in SS avout 17.00. Very quiet island. Pick either the 6 mile walk right round it, or the picture perfect beach or the shady square to read, doze and listen to the breeze in the trees. Local bar, cafe and hotel around to feed your needs whilst there as well.
Hope this helps....