Ah, Es Cana not Greece ! Regarding your enquiry about the medieval thingy,unless you have a hire car or get a taxi probably not viable,depending on how late your arrival is.The bus service from Es Cana is good imo,although you have to change at Santa Eularia on most trips,the problem being that in May it is not the full summer service,so less buses and earlier finish..:x
Trips and where to book ? I don't do this myself,but your hotel will almost certainly have a big rack of leaflets,that's if you like the organized type of stuff ?
There are a few places around that sell trips otherwise.
One thing I will say you should do is get on the boat down at the little harbour,go for a day to Ibiza town...
