Hi Sandy - where are you staying?
I have never seen any organised bus tours in the years I have been going to Ibiza, its not a huge Island, and I think most people do hire a car and drive around. There are some fabulous beaches, to find, the Island itself is very green with some lovely drives/walks up north to Portinax, San Miguel etc, there are old smugglers caves to visit at the North of the Island. Visiting Ibiza town and D'alt Villa are a must, the architecture and history of the town are fantastic, (not to be done in the middle of the day I hastened to add - its a good 20 min walk to the top of D'Alt Villa and its hot hot hot!!) If you look around this site you will find a whole host of things to see and to visit. Happy Hunting!!